The dictionary form of Japanese verbs always ends in a -u syllable. Ichidan (one row or single-step in German) verbs will always end in -る (-ru, e.g. 食べる, taberu, to eat) while godan (five rows or five-step in German) verbs can end in any of the following:
- -う (-u) as in 会う (au, to meet)
- -く (-ku) as in 書く (kaku, to write)
- -ぐ (-gu) as in 泳ぐ (oyogu, to swim)
- -す (-su) as in 話す (hanasu, to speak)
- -つ (-tsu) as in 立つ (tatsu, to stand)
- -ぬ (-nu) as only (!) in 死ぬ (shinu, to die)
- -ぶ (-bu) as in 遊ぶ (asobu, to play)
- -む (-mu) as in 読む (yomu, to read); or
- -る (-ru) as in 有る (aru, to exist/to be/to have)
It is notable that -う (-u) is actually from -wu, which can only be seen in the (w)a-form (used e.g. in negation 会わない rather than *会あない) as all other w- syllables have merged with their respective Ø- forms. Furthermore, it is interesting that among voiced/unvoiced pairs only -く (-ku) and -ぐ (-gu) coexist as verb endings; -ず (-zu) and -づ (-dzu) do not form verbs to the best of my knowledge and neither do -ふ (-fu) or -ぷ (pu). Verbs ending in -ゆ (-yu) or an -う (-u) which is not actually a historic -wu seem not to exist at all. Finally, according to my grammar book there is only a single verb that ends in -ぬ.
What are the origins of this perceived imbalance? How come only one voiced/unvoiced pair exists in contemporary Japanese and are there any mechanisms that explain the absence of the other missing syllables e.g. by mergers? And why is it -ぶ but -つ and -す?