Suru (to do) and kuru (to come) are highly irregular verbs, and none of their conjugations follow the standard patterns. Suru is called a sahen (サ変) verb in Japanese, and kuru is called a kahen (カ変) verb. There are no other sahen or kahen verbs.
| Verb form | suru | kuru |
|---|---|---|
| Negative | {{different|shi}}nai | {{different|ko}}nai |
| Past | {{different|shi}}ta | {{different|ki}}ta |
| Imperative | {{different|shi}}ro/{{different|se}}yo (formal) | {{different|koi}} |
| -te form | {{different|shi}}te | {{different|ki}}te |
Iku (to go) has irregular -te, -ta forms itte, itta: if it were regular, they would be iite, iita.
These honorific verbs all have an irregular stem such as gozaimasu: if they conjugated in the same way as other verbs they would conjugate as gozarimasu and so on.
Kureru has an irregular imperative form kure
Aru is mostly regular, but for its negative nai is used, not aranai.
Iu is irregular in its pronunciation, varying between yuu and iu.
Some suru verbs conjugate as two verbs, aisuru and aisu. aisu is used for some forms, including for instance the potential form aiseru instead of aidekiru. Similarly for verbs such as yakusu (translate).