Verb Groups
Japanese verbs are divided into three groups based on the last syllable of the basic (dictionary) form. You need to know which group a verb belongs to in order to produce the conjugated forms.
Grouping rules:
Group 1:
Verbs in group 1 end with the syllable ru (る), with the preceding syllable containing the vowels e or i. Some exceptions are kaeru (帰る "return home"), hashiru (走る "run"), kiru (切る "cut"), iru (要る, "need"), and hairu (入る "enter").
Group 2:
Verbs in group 2 end with syllables ku (く), gu (ぐ), su (す), mu (む), nu (ぬ), bu (ぶ), u (う), tsu (つ), or ru (る). Verbs ending in ru (る) contain the vowels u, o or a in the preceding syllable. Some exceptions are kaeru (帰る "return home"), hashiru (走る "run"), kiru (切る "cut"), iru (要る, "need"), and hairu (入る "enter").
Group 3:
Verbs in group 3 are suru (する "do") or kuru (くる "come").