The construction of a verbal system which is fairly regular and at the same time based on existing languages is a most difficult task, because in no other domain of the grammar do languages retain a greater number of ancient irregularities and differ more fundamentally from one another. Still an attempt will be made here to conciliate the two points of view and to bring about something which resembles the simple Chinese grammar without, however, losing its European character or the power of expressing nuances to which we are accustomed in our own languages.
One thing is common to all those schemes of constructed languages which have any importance, namely that endings to indicate person or number are nowhere found: person and number are shown by the subject, and the verb as such has logically nothing to do with them. This consideration helps us to get rid of a great many complications found in most of our European languages.
In the verbal forms Z sacrificed internationality to systematic a priori construction except in so far as he took the elements -nt- and -t- for his participles from national languages. Otherwise everything is built up a priori: infinitive in -i, imperative in -u, finite tenses ending in -s. The three chief tenses are distinguished by means of the vowels a, i, o, both in the finite forms (amas love, amis loved, amos will love) and in the participles (active amanta loving now, aminta having loved, amonta loving in the future, and correspondingly in the passive amata, amita, amota), to which comes a conditional amus would love (or `loved' in a clause beginning with if, etc.); it would have been possible to frame corresponding particles corresponding to this, amunta, amuta, but these find no place in Zamenhof's system. This play of vowels is not an original idea of Zamenhof's: -as, -is, -os are found for the three tenses of the infinitive in Faiguet's system of 1765; -a, -i, -o without a consonant are used like Z's -as, -is, -os by Rudelle (1858); Courtonne in 1885 had -am, -im, -om in the same values, and the similarity with Esperanto is here even more perfect than in the other projects, as -um corresponds to Z's -us.
In Ido this play of vowels was extended to the infinitive amar to love, amir to have loved, amor to love in future, to be going to love.
The chief objection to the whole system is that it is totally artificial without any connexion with our natural linguistic habits. Further it requires a distinct pronunciation in weakly stressed closed syllables, which is against the speech-habits of many nations (e.g. Englishmen, Russians). The six participles may be helpful in some instances to express nuances which ordinary languages do not distinguish; but they often also constitute a real embarras de richesse, especially in the passive, where one may often hesitate whether to say agitata or agitita, ornata or ornita (chambro orn. per flori), komplikata or -kita afero, restriktata or -tita, etc. Several teachers of Esp and Ido say expressly that these six participles constitute one of the greatest difficulties for their pupils (cf. also estus estinta `would have been').