The “Reconstructed Proto-Esperanto” language ("Vulgar Latin of Esperantia" or "Vulgar Latin of Ludovicia") is an attempt to hypothesize and reconstruct an ancestral language from the various Esperantidos (Esperanto-derived languages).
Esperanto version
Japanese version

Table of Contents

• Phonology
  • Consonants
  • Vowels
  • Accent
• Grammar
  • Case
  • Pronouns
  • Articles
  • Definite article
  • Indefinite article
  • Neuter indefinite (Partitive article)
  • Nouns adjectives
  • Neuter nouns
  • Declension
  • First declension
  • Second declension
  • Neuter declension
  • Verbs
  • Present tense
  • First conjugation
  • Second conjugation
  • Past tense
  • Irregular verbs
  • Future tense
  • Voluntative mood
  • Conditional mood
  • Perfect aspect
  • Progressive aspect
  • Durative aspect
  • Causative voice
  • Emphasis on verbs
  • Adverbs
• Esperantidos used for reconstruction
• Changes into Esperantidos
  • Phonological change ǝ to a in Romániço
  • Influence of Scholar’s Latin (Pseudo-Latin)

Phonology

Consonants

• Latin word-final tr changes to a retroflex consonant (either ɖ or ɖ͡ʐ). For convenience, it is written zr below.
• Pronunciations derived from Greek χ are kh or x.
• k and g before front vowels are palatalized to t͡ʃ and d͡ʒ. For convenience, they are written c and gx below.

Vowels

• Latin short close to close-mid vowels in word-final position are reduced to ə.
• The diphthongs ae and oe become æː and øː (short in final position).
• Both short and long vowels exist.

Accent

Accent combines both stress accent and pitch accent.
Stress accent is free and does not change when a word is inflected.
Pitch accent is fixed but may shift when a word changes form.

Grammar

The basic word order is SOV, but as in Latin, word order is free.
An emphasized phrase is often placed at the beginning.
There is a dedicated grammatical form for marking the topic.

Case

For words other than pronouns, the basic case inflections are nominative and oblique.
Other cases are expressed by postposed adpositions.

The nominative is used for subjects and vocatives.
The oblique is used for direct objects and for the notional subject of participles, infinitives, and gerunds.

The dative marks indirect objects and goals.
The partitive expresses “a part of…”, the object in a negative sentence, and the notional object of participles, infinitives, and gerunds.

Pronouns and nouns have “weak forms” used when followed by contracted articles or prepositions.
The ablative is used to express possession.

Pronouns

Due to substrate influence, the first and second persons have a dual number.

First person

Second person

Articles

The definite article is preposed when the noun is topical; otherwise, it is postposed.
When postposed, it is usually contracted to a definite ending.

Arcaicam Esperantom has only an indefinite article, which is unusual for a natural language.
This can be explained as a phenomenon that arose because the definite article is postposed and, due to sound change, the definite-article ending and the indefinite ending came to have the same sound.

The definite article cannot be used with neuter nouns.
Neuter nouns by themselves carry the same meaning as with a definite article; indefiniteness is expressed with the partitive.

Definite article

Forms combined with prepositions

ad takes the oblique in the singular and the nominative in the plural.

Indefinite article

Used only when topicalized.

Neuter indefinite

(Partitive article)

Nouns adjectives

There is no grammatical distinction between nouns and adjectives.
Nouns can be used adjectivally as they are, and adjectives can be used as nouns as they are.
When an adjective is used as a noun referring to an animate entity, it is treated as masculine or feminine; otherwise, it is treated as neuter.

When modifying a specific referent, an adjective takes the ablative if neuter, and the dative otherwise.
The inessive and illative also have adjectival uses, meaning “inside …” and “into …”.

Neuter

Neuter nouns include concepts, abstract nouns, mass nouns, uncountables, loanwords, and adjectives referring to inanimates.
The nominative and oblique are identical; in the plural, they follow the feminine singular pattern.

Declension

Declension is simplified from Latin into three classes: First Declension, Second Declension, and Neuter Declension.

First declension

The paradigm differs depending on whether the noun is animate.
It is a mixture of Latin First Declension and the feminine suffix -issa.
The oblique is influenced by the neuter plural partitive form.
For animates, it is mainly used for female terms.
Weak forms differ between nominative and oblique.

First declension animate

Definite endings

First declension inanimate

Definite endings

Second declension

Derived from Latin Second and Third Declensions.

This “Ø” means zero ending.

Definite endings

Neuter declension

Used for neuter nouns.

When indefinite, the oblique becomes partitive.

Verbs

The tenses are past, present, and future.
Aspects are perfect, progressive, durative, and prospective; voices are active and passive.
The present and future are not distinguished morphologically, nor are the past and present perfect; distinctions are made periphrastically with aspects.
Moods are indicative, subjunctive, voluntative, and conditional. The imperative uses the subjunctive.
There are active and passive infinitives.
The future active infinitive and future passive participle are expressed with the present active infinitive + inflected iːrǝ (“go”).
The future passive infinitive and future active participle are expressed with the present active infinitive + the copula essǝ (normally contracted).

Conjugation

There are First and Second conjugations.

Present tense

First conjugation

From Latin First Conjugation.

Present active infinitive -Ø

Present passive infinitive -arə

Gerund -ǝm

Future active infinitive -Ø em ad (-em ad)

Future passive infinitive -Ø forǝ (-orǝ)

Present theme -a-

Subjunctive theme -ə-

Future active participle -Ø fuːzrǝs (-uːzrǝs)

Future passive participle -Ø endǝs (-endǝs)

Present participle -antəs

Second conjugation

Derived from Latin Second–Fourth Conjugations.

Present active infinitive -Ø

Present passive infinitive -ərə

Gerund -ǝm

Future active infinitive -Ø em ad (-em ad)

Future passive infinitive -Ø forǝ (-orǝ)

Present theme -ə-

Subjunctive theme -a-

Future active participle -Ø fuːzrǝs (-uːzrǝs)

Future passive participle -Ø endǝs (-endǝs)

Present participle -əntəs

Past tense

The past tense is conjugated the same regardless of verb class. There is no past subjunctive.

(Latin 1st conj. -awi → -oj → -əj-, 2nd conj. -ui → -uj → -əj-, 3rd conj. -i → -ij → -əj-, 4th conj. -iwi → -uj → -əj-.)

Passive infinitive -əjssə

Active infinitive -əj

Past stem -əj-

Past participle -ǝjtǝs

Irregular verbs

Only essǝ (“to be”) and iːrǝ (“to go”).

essǝ conjugation

No present participle.

Present infinitive essǝ (stem ess-)

Future infinitive forǝ

Past infinitive fuǝ

Future participle fuːzrǝ (from Latin futurus)

Past participle fuǝs

iːrǝ conjugation

Future active infinitive = gerund + ad.

Active infinitive iːrə

Passive infinitive iːrərə

Gerund em

Future active infinitive em ad

Future passive infinitive iːrə forǝ (iːrorǝ)

Future active participle ezrǝs

Future passive participle endǝs

Present participle entəs

Past participle etəs

Although “to go” is intransitive, it has a passive form to restrict it to the intransitive meaning and to also express “to be continuously done”.

Future tense

Future events can be expressed with the present tense, but for clarity or emphasis use:

• Future infinitive + essǝ → simple future
• Future participle + essǝ → intentional future (“going to…”)

The subjunctive present can also mark imminent or probable future (prospective aspect).

The prospective participle is built from the future passive infinitive + iːrə present or past participle, usually contracted to -ontǝs or -otǝs.

Voluntative mood

In addition to the above intentional future,

• Subjunctive + essǝ → voluntative present progressive (“be about to…”)
• Infinitive + subjunctive iːrə → voluntative present durative (“be going to keep…”)

In first person, voluntative is standard; indicative in such cases implies reluctance.

Conditional mood

• Past participle + orə + es → conditional past
• Past active infinitive + uːzrǝ + es → conditional present

Perfect aspect

Use past participle of essǝ fuə (often contracted to infinitive + u).

Progressive aspect

Like English progressive: active infinitive + ad + essǝ.

Past progressive: use past form of essǝ.

Often contracted (-assum, etc.).

Progressive participles: active infinitive + ad + iːrə present/past participle → usually contracted to -antǝs, -atǝs.

Durative aspect

Ongoing/habitual: active infinitive + auxiliary iːrǝ.

Use past/future of iːrǝ for past/future habitual.

Causative voice

Infinitive + ag (second conjugation causative verb).

Emphasis on verbs

Use participle + essǝ to emphasize a verb.

Adverbs

When adverbs are derived from nouns or adjectives, the ablative forms -eks or -ellu are used. (This is the origin of Esperanto’s derived adverbs.)
Apart from that, there are forms that are originally adverbs. (This is the origin of Esperanto’s “special adverbs.”)
There is also a suffix -izr that expresses location or time and forms adverbs. (This is the origin of Esperanto -aŭ and of -ez in Archaic Esperanto.)

Esperantidos used for reconstruction

Aiola
Arcaicam Esperantom
Arlipo
Atlango
Esperanto
Farelix
Farlingo
Ido
Linguna
Mezepoka Esperanto (Sperantu)
Международно-Научный язык I
Mundolinco
Mondlingvo
Pra-Esperanto
・Lingvo Universala
・Lingwe Universala
Reformed Esperanto (Esperanto 1894)
Popido
Romániço
・Old Romániço
Uniëspo
Unitario

Virgoranto and Folkssem, thought to be of Germanic origin, are not included in the genealogical analysis but were used for reference.

Also, Volapük and Amaubo are considered to have had a close relationship with Esperanto, influencing Arcaicam Esperantom and Pra-Esperanto.

Changes into Esperantidos

In many languages, the change ǝ>o or ǝ>u occurs in word-final position, leading to a shift of the nominal ending to -o.

In many languages, the changes ǝ > o or ǝ > u, together with the erosion of definite-article endings into -o or -u, caused the two forms to merge and become indistinguishable. As a result, the suffixed definite article fell into disuse. In Arcaicam Esperantom, an indefinite article unn emerged, and the absence of an indefinite article came to mark definiteness. In Lingvo Universala, the preposed definite article originally used to indicate the topic—came to be used with subjects, while the postposed definite article came to be used as an accusative ending.

For the ending -a, the following changes are observed:
・-a remains unchanged as -a
・a>ǝ, then ǝ>o or ǝ>u (as in Arcaicam Esperantom)
・a>ǝ, but when followed by the definite article as in -aul, it changes to aː, preserving the -a form (as in Linguna and Mundolinco)
・change ǝ>a (see below)

In verbs, when the present stem undergoes ǝ>o, it collides with the contracted future stem -o- from the future infinitive. Since the present tense also covers the meaning of near future, it is absorbed into the future tense.

However, in Mundolinco it is preserved as a distinct present tense.

In Mezepoka Esperanto (Sperantu), ǝ>u occurs, the nominal ending becomes -u, and the verb present tense is absorbed into the subjunctive. And a>ǝ>u occurs, but when followed by the definite article as in -aul, it changes to -a. In some Esperantidos, like Mezepoka Esperanto, the ending -a occurs only with terms for women or female animals. This is the result of inanimate nouns becoming neuter, the definite article being limited to animate beings, and, subsequently, the aforementioned change, which led to the restriction of -a to human females and female animals.

With the loss of the present tense, the -a- stem from the contracted progressive aspect comes to mark the present tense.

However, in some languages, the -e- stem from the durative aspect comes to mark the present tense (in Lingvo Universala, -a- changes to mark the past progressive).

Amid the confusion of conjugations, the paradigm is simplified to a single pattern.

The past tense undergoes ǝj>i, yielding a past stem in -i-, but in Mezpoka Esperanto (Sperantu) ǝj>aj.
Some languages have -av-, -u-, -avi- for perfect or past, derived from the form infinitive + u marking the perfect aspect.

In Esperantidos, the first person singular often ends in -m, either from using the subjunctive for first person subjects or from the ending of sum.
In Arcaicam Esperantom and Linguna, the 1sg endings are -ms and -ym (only in present for Linguna), arising from contraction: 1sg ending contracts to s from sum, then to avoid collision with 2sg -s, another form is introduced.
In Arcaicam Esperantom, this comes from the voluntative present progressive (subjunctive + esǝ), possibly by analogy to the 1pl.
In Linguna, it comes from the present habitual ewm.

Contraction of endings also affected case endings:
・Oblique becomes -n, absorbing the partitive (in many languages)
・Partitive shifts to dative meaning
・Partitive shifts to genitive
・Oblique becomes instrumental, partitive becomes accusative (in Arcaicam Esperantom, instrumental further shifts to nominative)

Adjectives come to be marked only by forms from dative -al (modifying specific nouns) or from ablative -el, leading to a distinction between nouns and adjectives.
As dative meaning spreads to adjectives, other cases are repurposed for the dative: from allative in Arcaicam Esperantom, from illative in Linguna (also the partitive > dative path above).

Agglutinative shift

Arcaicam Esperantom and Lingvo Universala may have become more agglutinative under the influence of Volapük. However, due to major phonological differences, Arcaicam Esperantom might instead descend from a lost dialect of Volapük.

Phonological change ǝ to a in Romániço

In Romániço, word-final ǝ changes to a.

As a result, the present and present progressive merge, reducing verb conjugation to one pattern.

In nouns, the ending became -a, but it merged with the adjectival ending -a derived from the dative -al, making the two indistinguishable. As a result, nouns with a suffixed definite article took the form -au, while those without a definite article took -a; adjectives, whether definite or indefinite, also took -a, creating a complex system. This was later simplified: the form with the suffixed definite article (-au, later changing to -o) came to be treated as a noun, while the form without the definite article came to be treated as an adjective. This holds even with a preposed article, and survives in modern Romániço as the “article + adjective” pattern for nominalized adjectives.

Consonant weakening at word-final position was strong, so case inflection disappeared early, and person marking in verbs simplified.
Verb person marking derives from 3pl forms (and also from the participle + essǝ used for emphasis).
All cases except nominative were unified as -n, which disappeared by the medieval period.

Influence of Scholar’s Latin

(Pseudo-Latin)
The Esperanto-speaking region had a distinctive pronunciation of Latin, called “Scholar’s Latin” or “Pseudo-Latin”.

Among Latin-derived vocabulary in Esperanto-derived languages, some words underwent phonological change like in the Romance languages, while others preserved the Latin form.
In Esperanto and others, many words have stress patterns differing from Latin.
While direct borrowing from Latin is possible, many such words are not “high-register” vocabulary.

Thus, it is hypothesized that there was a regional Latin variety functioning as a lingua franca among dialects.
Latin has long had region-specific pronunciations (e.g., Italian, German, and now little-used English or French styles).
It is proposed that the unchanged Latin vocabulary in these languages was pronounced in a regional “Esperanto-style” Latin, serving as a lingua franca and thus borrowed into everyday language.

Stress is usually influenced by regional speech patterns, and minor stress shifts would not disrupt mutual comprehension, so these Latin loans entered with stress positions differing from Classical Latin.

This explains languages that use only unchanged Latin vocabulary: these are literary, using Latin vocabulary for most content words except inflected forms, conjugations, and function words.

Esperanto and many related languages have extensive vocabulary created with the mal- prefix and fine-grained morpheme compounding. While unnatural for a natural language, this may have flowed from strict literary usage norms.

In summary:

In this region, contact with Germanic languages eroded Latin endings and altered grammar, producing the Proto-Esperanto described above.
Latin was also pronounced with reduced or eroded inflectional endings.
Among the elite, Latin was used in speech; later, as dialects diversified, Latin served as a lingua franca.

However, erosion of endings made Latin inflection indistinct, so speakers adopted the inflectional endings of Romániço (which may have been the capital’s language).
This spread Latin vocabulary alongside Romániço grammar (single verb conjugation type, clear noun–adjective distinction, simplified case system) as common features of Esperantidos.

Later, “scholars” codified this literary style, emphasizing mal- derivatives and complex compounds. (Languages lacking such vocabulary were outside this “school’s” influence.) This is “Scholar’s Latin”.

In later periods, dialects were also written down, mixing local grammatical morphemes and function words with Latin (Scholar’s Latin) vocabulary.
By then it was no longer Latin but “Pseudo-Latin”.

In modern times, writing became more colloquial, but Esperanto retained features of Scholar’s Latin when becoming the standard.

Many Esperantidos have names meaning “world language”; originally an honorific for Latin, this title shifted to local language names as Latin spread in spoken form.