To form the plural of substantives, adjectives and pronouns, Zamenhof took the letter -j, pronounced like E y, and thus created the often criticized forms chiuj tiuj bravaj homaj agoj, `all those brave human acts,' etc., in the accusative chiujn tiujn bravajn homajn agojn. The device is not taken from any living language, though it has a kind of prototype in ancient Greek. If we want to go to living languages to find a plural ending, we may leave aside -r, which is used to some extent in Sc, -e, which is found in D Sc, -en or -n, which is pretty frequent in D and Dutch (though in the latter language generally mute); and the choice thus is restricted to -i and -s. The ending -i is frequent in I R, whence it is known in E in a few loan-words: banditti, bolsheviki. This ending was selected in Ido (with omission of the substantival ending of the singular, thus homi from homo man), the reason being twofold: a vocalic ending was wanted in order to make the addition of the (Esp) accusative -n possible, and on the other hand the s-endings were used as in Esp for the verb. But any one whose choice is not determined by these two reasons is sure to prefer the ending -s, as it is much more international, being the regular mark of the plural in such important languages as E S P (F spelling), besides being used in an increasing number of words in D (kerls, autos, jungens) and Dutch.
Examples with various vowels before -s: home homes, homo homos, homa homas, temple temples, kasu kasus, alkali alkalis.
The same rule applies to adjectives as primaries (substantivized): bones, bonos, bonas (good ones, good men, good women), and also to pronouns treated in the same way: omnes omnos omnas, altres altros altras; tes those, with tos m., tas f., etc.
Thus even with personal pronouns, which are more or less irregular in other constructed languages, but are here beautifully regular: vu you (speaking to one person), pl. vus; les they, with the two-sex forms los and las. The only irregularity is found in the first person, where it is justified by the fact that "we" does not mean several "I's," but "I + someone else or several others," therefore me I, nus we. (If a king should ever speak in Novial, he might say nu as the singular of nus, a "pluralis majestatis in singulari.")
Secondary words (predicate verbs and adjuncts) never require any plural mark: li altri femas esed oldi the other women were old.
NOTE.- Z quite properly gave to his definite article the same ending as to his adjectives: la bona, but he also felt that it would be unsupportable to inflect it like adjectives (lajn bonajn homojn etc.) and therefore made la invariable: Ido was more consistent and made adjectives invariable too (la bona homi). But then Ido had difficulties with "substantivized adjectives" (adjectives as primaries), and there invented the unlucky device of inflecting the article, plural le bona, neuter lo bona.
Indefinite number.- A Japanese Idist once told me that his countrymen (and the Chinese as well) missed in the proposed international languages a form which was neither singular nor plural, and thus left the number indefinite. Now it is easy in N to make such forms by simply leaving out the final vowel, thus hom would mean "man or woman or men or women"; this would be particularly useful in the generic sense (i.e. when speaking of the whole species), e.g. leon es kruel, where we say indifferently `the lion is cruel' (or `a lion is cruel') or `lions are cruel.' Note further the following sentences in which we might use the indefinite number: Hir vort manka, a word, or some words, wanting here. Me non pove ama hom kel non estima me, I cannot love a man, or men, who does not (do not) esteem me. Les vadad a hem, they went home, i.e. each to his or her own home, or all to their common home. Yet I suspect that Europeans and Americans will make a very sparing use of this indefinite form. With the reflexive pronoun se (D sich) sex and number are always shown by the subject of the sentence and therefore require no formal indication: lo admira se he admires himself; la admira se she admires herself; les (los, las) admira se they admire themselves.
The uninflected article and adjuncts may be mentioned also as examples of the indefinite number.