できる: The Suppletive Potential of する | Japanese Grammar

Japanese builds the potential ("can / be able to") by regular rules for almost every verb — 話す → 話せる, 食べる → 食べられる. But the most frequent verb of all, する, does not play along. Its potential is a suppletive form: a completely different word, できる. You cannot derive it, and — crucially — it is not される (that is the passive) or せられる (a stiff classical form). This page shows how できる works as する's "can," how it conjugates, and how it doubles as the everyday verb for "be finished / come into being."

できる is a regular 一段 verb

The good news: once you have できる, it inflects like any 一段 verb — no sound changes, no surprises. Chop the る, add the ending.

Form	できる	Reading
Dictionary	できる	dekiru
Polite 〜ます	できます	dekimasu
Negative 〜ない	できない	dekinai
Polite negative	できません	dekimasen
Past 〜た	できた	dekita
Past-negative	できなかった	dekinakatta
Te-form 〜て	できて	dekite
Conditional 〜ば	できれば	dekireba
Conditional 〜たら	できたら	dekitara

Because a potential already means "can," you do not re-potentialize it: there is no everyday ×できられる. Likewise できる has no ordinary passive or volitional — it is a stative "can/come-to-be" verb, so those slots simply aren't used.

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できる ends in -きる, and the き sits on the i-row, so it is a genuine 一段 verb: negative できない, past できた, te-form できて. Do not godan-ise it — ×できらない and ×できった are the classic slips.

The core job: the potential of する

Whenever you want to say "can do" for a する verb, you reach for できる. The thing you are able to do is marked with が, not を — that is the general rule for potential objects.

彼女はフランス語も中国語もできる。

kanojo wa furansugo mo chūgokugo mo dekiru

She can speak both French and Chinese.

静かだから、ここなら集中して勉強できる。

shizuka da kara, koko nara shūchū shite benkyō dekiru

It's quiet, so I can study with focus here.

ごめん、明日はどうしても参加できない。

gomen, ashita wa dōshite mo sanka dekinai

Sorry — there's no way I can make it tomorrow.

The anchor pattern is 〜ができる with a noun — a skill, a language, a task — meaning "can do / be capable at it":

日本語ができると、旅行がぐっと楽になるよ。

nihongo ga dekiru to, ryokō ga gutto raku ni naru yo

Once you can speak Japanese, travel gets so much easier.

Every 〜する compound potentializes as 〜できる

This is the payoff. Because noun + する verbs conjugate on the same する, their potential is formed by swapping する → できる. The noun does not change.

予約する → 予約できる（can reserve）
運転する → 運転できる（can drive）
説明する → 説明できる（can explain）
理解する → 理解できる（can understand）

このアプリなら、深夜でもタクシーを予約できる。

kono apuri nara, shin'ya demo takushī o yoyaku dekiru

With this app you can book a taxi even in the middle of the night.

事故のあと、しばらく運転できなかった。

jiko no ato, shibaraku unten dekinakatta

After the accident I couldn't drive for a while.

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できる is the compact everyday potential. The heavier periphrastic form 〜することができる means the same thing — 予約することができる = 予約できる — but is longer and a touch more formal. See する + ことができる.

The pitfall: できる, never ×される or ×せられる

Here is the error the whole page exists to prevent. Having just learned that 一段 verbs make the potential in 〜られる, English speakers try to build a potential of する the same way and produce something that already has a meaning — the wrong one.

される is the passive of する: "is done / gets done (to someone)."
せられる is a stiff, classical causative-passive; modern Japanese uses させられる ("be made to do").
Neither means "can." The potential is できる, full stop.

この部屋はネットで予約できる。

kono heya wa netto de yoyaku dekiru

You can book this room online. (potential)

この部屋はもう予約された。

kono heya wa mō yoyaku sareta

This room has already been booked. (passive — a completely different meaning)

Read those two side by side and the danger is obvious: 予約できる and 予約される are not variants of one idea — they are "I am able to book" versus "it got booked."

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Burn this in: する's "can" is できる. If you catch yourself writing 〜される or 〜せられる to mean "be able to," stop — される is what happens to something, not what you are able to do.

The other できる: "be completed, come into being, be ready"

できる is written 出来る — literally 出（で, come out）+ 来る（くる, come）— and its original meaning is "to come into being, to be produced, to be finished." That older sense is alive and everywhere, and it is how the potential meaning arose (roughly: "it comes about that X can be done" → "X is possible"). In this use できる is intransitive and describes something appearing, finishing, or forming.

ごはんできたよ、冷めないうちに食べて。

gohan dekita yo, samenai uchi ni tabete

Dinner's ready — eat it before it gets cold.

駅前に大きなショッピングモールができた。

ekimae ni ōkina shoppingu mōru ga dekita

A big shopping mall went up in front of the station.

実は、子供ができたんです。

jitsu wa, kodomo ga dekita n desu

Actually, we're expecting a baby.

The でできている pattern ("be made of / consist of") is the same verb — the resultant state of "having come into being from":

この橋は主に木でできている。

kono hashi wa omo ni ki de dekite iru

This bridge is made mainly of wood.

Two frozen phrases worth memorizing

できる anchors two set expressions you will use constantly:

できれば — "if possible / if you can" (formal-neutral, very common in requests).
できるだけ — "as much as possible / as ... as you can."

できれば、今日中にお返事をいただけますか。

dekireba, kyōjū ni o-henji o itadakemasu ka

If possible, could I have a reply by the end of today? (polite request)

できるだけ早く来てね。

dekiru dake hayaku kite ne

Come as soon as you can, okay?

In stiff business Japanese, the negative "cannot / unable to" is often softened to できかねます (formal) — 「その件は、お答えできかねます」("I'm afraid I can't answer that one") — but that is a register upgrade of できない, not a different verb.

Common mistakes

❌ 私はまだ日本語がされない。

Incorrect (intending 'I can't speak Japanese yet') — される is the passive. The potential of する is できる: 日本語ができない.

✅ 私はまだ日本語ができない。

watashi wa mada nihongo ga dekinai

I can't speak Japanese yet.

❌ 疲れて、もう運転せられない。

Incorrect — せられる is the classical causative-passive, not a potential. Use できる: 運転できない.

✅ 疲れて、もう運転できない。

tsukarete, mō unten dekinai

I'm tired — I can't drive anymore.

❌ 私は料理をできる。

Incorrect — the object of a potential takes が, not を: 料理ができる.

✅ 私は料理ができる。

watashi wa ryōri ga dekiru

I can cook.

❌ 昨日、やっと宿題ができった。

Incorrect — できる is 一段, so the past is できた (no っ). ×できった godan-ises it.

✅ 昨日、やっと宿題ができた。

kinō, yatto shukudai ga dekita

I finally finished the homework yesterday.

Key takeaways

できる is the suppletive potential of する — "can do." It is never ×される (passive) or ×せられる (classical).
It is a regular 一段 verb: できる・できます・できない・できた・できて・できれば — no sound changes, and no ×できった.
The thing you can do is marked with が: 日本語ができる, 料理ができる.
Every 〜する compound potentializes by swapping する → できる: 勉強できる, 予約できる, 運転できる.
The same verb 出来る means "be completed / come into being / be made of" — ごはんができた, 子供ができた, 木でできている — the older sense that gave rise to the potential.

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