The phonology of the language is very simple. The vowels are only five in number, a, e, i, o, u, used without any distinction of quantity, as in Russian. There are six diphthongs, expressed by an unnecessarily complicated notation. The consonant-system is simple enough in itself, but is greatly complicated in writing by the excessive and mostly unnecessary use made of diacritical letters not only for simple sounds but also for consonant-groups. c is used for ts, as in Polish.
The grammar is, like that of Volapük, partly borrowed from existing languages, partly a priori and arbitrary. The use of the final vowels belongs to the latter category. The use of -a to indicate adjectives and of -o to indicate nouns as in kara amiko, “dear (male) friend,” is a source of confusion to those familiar with the Romance languages, and has proved a bar to the diffusion of Esperanto among the speakers of these languages. On the other hand, the following paradigm will show how faithfully Esperanto can reproduce the defects of conventional European grammar:—
| Singular. | Plural. | |
| Nominative | la bona patro | la bonaj patroj |
| Accusative | la bonan patron | la bonajn patrojn. |
It is difficult to see why the accusative should be kept when all the other cases are replaced by prepositions.
The verb is better than the noun. Its inflections are -as present, -is preterite, -os future, -us conditional, -u imperative and subjunctive, -i infinitive, together with the following participles:—
| Active. | Passive. | |
| Present | -anta | -ata |
| Preterite | -inta | -ita |
| Future | -onta | -ota |
The inventor has followed the good example of his native language in using esti, “to be,” as the auxiliary verb both in the passive, where it is combined with passive participles, and in the secondary tenses of the active (perfect, pluperfect, &c.), where it is of course combined with the active participles. The participles can be made into nouns and adverbs by changing the final -a into -o and -e respectively: thus tenonto, “the future holder,” perdinte, “through having lost.”
The table of the forty-five correlative pronouns, adjectives and adverbs is also elaborate and ingenious.
Much ingenuity is displayed in the syntax, as well as some happy simplifications. But, on the other hand, there is much in it that is fanciful, arbitrary and vague, as in the use of the definite article—where the author has unfortunately followed French rather than English usage—and in the moods of the verb.
The following specimens will show the general character of this easy-flowing but somewhat heavy and monotonous language—“bad Italian,” as it is called by its detractors:—