The irregular verbs: する and くる (サ変 & カ変)
Master the only two true irregular verbs in Japanese: する (to do) and 来る (to come). Learn how する powers hundreds of noun compounds, how 来る shifts its kanji reading, and why their forms unlock advanced grammar.
The Two Irregular Pillars: する and くる
suru (to do) · kuru (to come)
Japanese has strictly two irregular verbs. Master their stems and you have mastered every irregular verb in the language.
The Two Irregulars: Compound Power vs Sound Shift
Micro-Shift日本語を 勉強します。
Noun + する creates hundreds of active verbsI study Japanese.
する pairs with Sino-Japanese and foreign action nouns to form countless everyday verbs.
Example Sentences
Parakeets mimic words
How it works
One of Japanese’s two irregular verbs: する (to do) becomes 真似する (to mimic) when attached to nouns.
Mental Model: The Two-Verb Blessing
In European languages like English, French, or Spanish, learners must memorize hundreds of irregular verbs with chaotic sound changes (go → went → gone, be → was → been, catch → caught).
Japanese offers an extraordinary grammatical gift: there are strictly two irregular verbs in the entire language:
- する (“to do”) — Grammatically classified as サ行変格活用 (sa-row irregular).
- 来る (“to come”) — Grammatically classified as カ行変格活用 (ka-row irregular).
Every other verb in modern Japanese—without a single exception—follows regular Godan or Ichidan rules.
1. する: The Universal Verb Engine
する is the most productive verb in the language because it turns any Sino-Japanese noun or foreign loanword into an instant action verb:
- Noun + する:
- 勉強 (study) + する = 勉強する (“to study”)
- 料理 (cooking) + する = 料理する (“to cook”)
- 旅行 (travel) + する = 旅行する (“to travel”)
- ドライブ (drive) + する = ドライブする (“to go for a drive”)
The core conjugation spectrum of する:
- Dictionary form: する (“to do”)
- Polite form: します (“do / will do”)
- Plain negative: しない (“do not do”)
- Plain past: した (“did”)
- Te-form: して (“doing / do and...”)
- Potential form: できる (“can do”)
2. 来る: The Shifting Kanji Chameleon
The challenge of 来る is purely visual: the kanji 来 stays the same, but its reading shifts across vowel rows:
- Dictionary form: 来る (vowel u)
- Polite form: 来ます (vowel i)
- Plain negative: 来ない (vowel o — never *kinai!)
- Plain past: 来た (vowel i)
- Te-form: 来て (vowel i)
Common compound verbs built with 来る:
Foreigner Traps to Avoid
Conjugation
| Base form | Conjugated | Form |
|---|---|---|
| する | します | Polite affirmative (do) |
| する | しない | Plain negative (not do) |
| 来る | 来ます | Polite affirmative (come) |
| 来る | 来ない | Plain negative (not come) |
In Dialogue
Kenji
Yuki
Tips and Common Traps
Quick Check
What is the correct plain negative form of 来る?