The -ιά Feminine: βαθύς and γλυκός

Two adjective classes share one distinctive feminine, and meeting them together is what makes them easy. The first is -ύς / -ιά / -ύ: βαθύς, βαθιά, βαθύ (“deep”), and likewise πλατύς, βαρύς, μακρύς, ελαφρύς. The masculine and neuter are short in the nominative and accusative - βαθύς, βαθύ - and everything else in the paradigm runs on a -ι- stem: βαθιού, βαθιοί, βαθιούς, βαθιών. Watch one syncretism: the feminine singular and the neuter plural are the SAME word, βαθιά. The second class is -ός / -ιά / -ό, whose masculine and neuter decline exactly like καλός and whose feminine SINGULAR alone breaks away: γλυκός, γλυκιά, γλυκό, with genitive γλυκιάς. Its plural then reverts to the ordinary -ές / -ών, so it is γλυκές and never «γλυκιές» - that asymmetry is the one thing to get right. κακός and μαλακός carry the same -ιά feminine as a secondary form beside their regular -ή.