Verb conjugates are often grouped into consonant-stems (五段動詞 godandōshi) (type I) and vowel-stems (一段動詞 ichidandōshi, いる, iru and える eru forms) (type II). The plain form of a type I verb has an う u sound (u, tsu, ru, ku, gu, nu, bu, mu, su), the ~ます -masu form has an い i sound (i, chi, ri, ki, gi, ni, bi, mi, shi), and the negative form has an あ a sound (wa, ta, ra, ka, ga, na, ba, ma, sa). The potential form has an え e sound (e, te, re, ke, ge, ne, be, me, se) and the volitional form has an おう ō sound (ō, tō, rō, kō, gō, nō, bō, mō, sō). (wikipedia)
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