The chief use of adjectives is to serve as "secondary" words, i.e. either as adjuncts (epithets, attributive) or as predicatives. As number and sex are always indicated with sufficient clearness in the "primaries", it is superfluous to indicate them in secondaries. This rule is carried through in E and must be ours in N (as Ido, but different from Esp): natural formes; ti forme es natural; ti formes es natural, etc.
It has already been remarked that the bare stem-form of many adjectives would not be euphonious enough, and that it is therefore often convenient to have a vowel added to it: i is the best for that purpose, and we therefore have such N adjectives as simpli, nigri black, posibli, stranji, singli, propri own, etc. Even where the adjective ends in only one consonant, the same -i may optionally be appended to it for euphony, especially before words beginning with consonant groups; thus we have alternative forms like bon boni good, kruel krueli, matur maturi, sam sami, publik publiki, etc.
The choice of i for this purpose is justified (besides by the fact that a, e, o are otherwise occupied) by many derivatives like simpli-fika = fika simpli (below, p. 137), modernisa.
In some adjectives -i corresponds to E -ic (-ical): elektri, identi, kosmopoliti, fanati, etc.
When adjectives are made into primaries, we have the endings already considered, for persons -e, -o, -a, and for neuter -um. If the adjective is used by itself with reference to a substantive just mentioned ("anaphorically") the ending -i is retained, and a plural may be formed in -is: hir es du roses, li blanki es plu bel kam li redi the white one ... the red one. Hir es multi roses: ob vu prefera li blankis o li redis? Inter lon libres es multis in anglum e kelkis in latinum, ma lo lekte nur li anglis. Similarly: li maxim oldis ek lon libres the oldest ones among his books.
The definite article is a secondary and therefore uninflected in number or gender; its proper form should be the single sound l, but this is impracticable except in close connexion with some of the prepositions: al, del, dal (prol, pril are not so good); in all other cases we add -i, so that the usual form of the article is li (= Occ).