Definition:
Japanese verbs are divided into conjugation classes based on their stem shape and the alternations they undergo: Group 1 (godan verbs, also called u-verbs or consonant-stem verbs) have stems ending in a consonant that appears in different vowel grades before suffixes; Group 2 (ichidan verbs, also called ru-verbs or vowel-stem verbs) have stems ending in a vowel that remains invariant across conjugations; and a handful of irregular verbs (する suru “do” and くる kuru “come”) form a third class. Correctly identifying a verb’s class is essential for producing all conjugated forms — dictionary form (non-past), past tense, te-form, conditional, volitional, and so on.
In-Depth Explanation
Group 1: Godan Verbs (u-verbs / 五段動詞)
Godan (“five-step”) verbs have dictionary forms ending in a consonant + u: kaku (書く, write), yomu (読む, read), hanasu (話す, speak), kau (買う, buy), matsu (待つ, wait), shinu (死ぬ, die), tobu (飛ぶ, fly), nomu (飲む, drink).
The stem alternates across five vowel grades (a, i, u, e, o) before suffixes:
| Conjugation | Stem vowel | Example (kaku) |
|---|---|---|
| Negative (-nai) | -a | kakanai |
| Stem/ます (-masu) | -i | kakimasu |
| Dictionary (non-past) | -u | kaku |
| Conditional (-eba) | -e | kakeba |
| Volitional (-ō) | -o | kakou |
The te-form and past tense in godan verbs involve euphonic changes (音便 onbin):
- kaku → kaite (k→i+te, consonant deletion)
- yomu → yonde (m→nde, nasal assimilation)
- hanasu → hanashite (s→sh alternation)
- kau → katte (w→tt gemination)
Group 2: Ichidan Verbs (ru-verbs / 一段動詞)
Ichidan (“one-step”) verbs have dictionary forms ending in -iru or -eru and have a single invariant stem: taberu (食べる, eat), miru (見る, see), okiru (起きる, wake up), neru (寝る, sleep).
The stem is simply the verb minus final -ru:
- taberu → stem tabe-: tabe-nai, tabe-masu, tabe-ru, tabe-reba, tabe-yō, tabe-te
Ichidan verbs are more regular and easier to conjugate, but the difficulty for learners is identifying which verbs are ichidan vs. godan.
The Ambiguity Problem
Many verbs ending in -iru or -eru are godan, not ichidan:
- kaeru (帰る, to return home): GODAN — kaette, kaeri-masu
- kiru (切る, to cut): GODAN — kitte, kiri-masu
- kiru (着る, to wear): ICHIDAN — kite, ki-masu
- hashiru (走る, to run): GODAN
These must be memorized individually; the phonological form alone does not distinguish them.
Irregular Verbs
- する (suru, to do): shi-masu, shi-nai, shite, sureba, shiyō
- くる (kuru, to come): ki-masu, ko-nai, kite, kureba, koyō
Pedagogical Naming Conventions
Different curricula use different terms:
- Godan / ichidan — traditional Japanese grammatical terminology
- u-verb / ru-verb — common in English-language Japanese pedagogy (based on dictionary-form ending)
- Group 1 / Group 2 / Group 3 — JLPT/textbook terminology (Genki, Minna no Nihongo)
Common Misconceptions
“All verbs ending in -iru or -eru are ichidan.” This is a common learner mistake. A significant minority of -iru and -eru verbs are godan and must be learned as exceptions. Textbooks typically provide lists, but the only reliable method is encountering the te-form or past form to verify.