Japanese Irregular Verbs: する and くる — All Conjugations (2026)

Good news: Japanese has only two irregular verbs — する (suru, "to do") and くる (kuru, "to come"). Every other verb in the language follows one of two predictable patterns. Bad news: these two verbs appear constantly in everyday Japanese, and their forms are genuinely unpredictable. This guide gives you every conjugation form for both, plus the 15 most common compound する verbs you need to know.

Why する and くる are irregular

Standard Japanese verbs split into two groups. Group 1 (u-verbs / godan) end in a consonant + u sound (書く、飲む、話す). Group 2 (ru-verbs / ichidan) end in -eru or -iru (食べる、見る). Both groups conjugate with consistent stem changes.

する and くる fit neither pattern. Their stems shift in ways that cannot be derived from any rule — you simply have to memorise each form. The upside is that once you know them, you unlock hundreds of compound verbs instantly.

する (suru) — complete conjugation table

くる (kuru) — complete conjugation table

The stem changes at a glance

The biggest challenge with these verbs is that the stem itself changes — not just the ending. Here is what you need to watch:

• する stem: し (shi) in most forms, さ (sa) in passive/causative, せ (se) in one imperative, す (su) in the conditional すれば
• くる stem: き (ki) in polite forms and past, こ (ko) in negative and volitional, く (ku) in dictionary and conditional くれば

Compound する verbs — 15 essential examples

The real power of する is that it turns nouns into verbs. Attach する to any noun and you create a new verb that conjugates exactly like する. Here are 15 you will encounter immediately:

Common mistakes with する and くる

1. してる vs している — dropping the い

In casual speech, Japanese speakers often drop the い from ている. While natural in spoken Japanese, it is incorrect in formal writing.

• ✗ レポートを書いてる (spoken/casual) — not wrong in conversation
• ✓ レポートを書いています (correct written/formal form)
• ✗ 何してる？ (casual) — fine for texting a friend
• ✓ 何をしていますか (polite, correct in any context)

2. こない vs くない — negative confusion

Learners sometimes try to negate くる the way you negate い-adjectives (by adding くない), which is completely wrong.

• ✗ くくない — this is not a word
• ✗ くるない — incorrect negative attempt
• ✓ こない — the correct plain negative of くる
• ✓ きません — the correct polite negative of くる

3. してください vs してもらう — request forms

Both express requests but from different perspectives. してください asks the listener directly. してもらう (or してもらえますか for politeness) frames the request as "would you do me the favour of..." and is softer.

• ✓ 説明してください — Please explain (direct request)
• ✓ 説明してもらえますか — Could you explain for me? (softer, more polite)
• ✓ 説明していただけますか — Could you kindly explain? (very polite, uses いただく)

4. できる is not the potential of する the way you expect

Most verbs form the potential by changing the final vowel (書く → 書ける). する is special: its potential form is できる, which is actually a separate verb historically. You cannot say しられる for "can do."

• ✗ しられる (wrong potential of する)
• ✗ しれる (also wrong)
• ✓ できる — the correct potential form, meaning "can do" or "is possible"

Real sentences using する and くる

• 毎日日本語を勉強しています。
  Mainichi nihongo wo benkyou shite imasu.
  I study Japanese every day.
• 友達が明日うちにくる。
  Tomodachi ga ashita uchi ni kuru.
  My friend is coming to my place tomorrow.
• 彼は何もしなかった。
  Kare wa nanimo shinakatta.
  He did nothing.
• 電車が来なかったので、遅刻しました。
  Densha ga konakatta node, chikoku shimashita.
  Because the train didn't come, I was late.
• 日本語ができるようになりたい。
  Nihongo ga dekiru you ni naritai.
  I want to become able to speak Japanese.

Practice your Japanese with instant feedback

The best way to internalise する and くる is to use them in real sentences and get corrected immediately. Try ZISTICA MOJIIQ's free grammar checker — paste any Japanese you write and get AI-powered corrections with explanations of exactly which verb form went wrong and why.

Frequently asked questions