English has exactly one way to say "can": the little modal can, bolted in front of an unchanging verb. Japanese has two. You can conjugate the verb into the short potential — 泳ぐ → 泳げる ("can swim") — or you can leave the verb in its plain dictionary form and add the frame 〜ことができる: 泳ぐことができる. Both mean "can swim." This page is about the second route, the analytic or periphrastic potential — what it is built from, and the one thing that actually decides which of the two you should reach for: register, not meaning.
How it is built: nominalize the verb, then say it is possible
The construction has three transparent pieces, and seeing them is the whole point:
- こと — the nominalizer. It packages a whole verb phrase into a noun-like event or fact: 泳ぐ ("swim") → 泳ぐこと ("swimming / the act of swimming").
- が — the subject particle, marking that event as the subject.
- できる — the verb "is possible / can be done."
So 泳ぐことができる is, literally, "the act of swimming is possible (for me)." できる is doing the real work — it is the same できる that means "be possible" as a standalone verb, and the same one that pairs with plain nouns: 料理ができる ("can cook," literally "cooking is possible"). The verb version simply nominalizes first with こと so that a verb can occupy the slot a noun would.
毎日練習すれば、誰でも泳ぐことができるようになりますよ。
mainichi renshū sureba, dare demo oyogu koto ga dekiru yō ni narimasu yo
If you practice every day, anyone can become able to swim.
このアプリを使えば、世界中の新聞を無料で読むことができます。
kono apuri o tsukaeba, sekaijū no shinbun o muryō de yomu koto ga dekimasu
With this app, you can read newspapers from all over the world for free.
Because the whole weight of the tense and politeness sits on できる (itself an ichidan verb), you conjugate that: ことができます (polite), ことができない ("cannot"), ことができた ("was able to"), ことができるようになった ("became able to").
半年の練習で、ようやく自分の名前を漢字で書くことができるようになった。
hantoshi no renshū de, yōyaku jibun no namae o kanji de kaku koto ga dekiru yō ni natta
After half a year of practice, I finally became able to write my own name in kanji.
The object stays を — a crucial contrast with the short potential
Here is a difference that trips learners who have just learned the short potential's が-object. The short potential tends to re-mark its object to が because the whole form is stative: 日本語が話せる ("Japanese is speakable-to-me"). But in 〜ことができる, the verb is still an ordinary transitive dictionary form sitting inside the nominalized clause, so it keeps its normal を object. The が in the construction belongs to こと, not to the object.
私は日本語を話すことができます。
watashi wa nihongo o hanasu koto ga dekimasu
I can speak Japanese.
ここで写真を撮ることができます。
koko de shashin o toru koto ga dekimasu
You may take photographs here.
Compare the two "can speak Japanese" sentences directly:
| Route | Sentence | Object particle |
|---|---|---|
| Short potential (stative) | 日本語が話せる | が (object re-marks) |
| ことができる (analytic) | 日本語を話すことができる | を (stays as normal object) |
The real difference: register, not meaning
泳げる and 泳ぐことができる mean the same thing. What separates them is weight and register. The short potential is light, quick, and dominant in conversation. The analytic potential is heavier and more explicit, and it rules the written and formal world: signage, rules, instructions, official announcements, application requirements, textbooks, and careful speech.
Think of it as the difference between can and is able to / is permitted to in English — both correct, but the longer one is what you write on a sign.
図書館では、一度に五冊まで本を借りることができます。
toshokan de wa, ichido ni gosatsu made hon o kariru koto ga dekimasu
At the library, you may borrow up to five books at a time.
応募資格：三キロ以上泳ぐことができる方。
ōbo shikaku: san-kiro ijō oyogu koto ga dekiru kata
Eligibility to apply: those who can swim three kilometers or more.
In the negative, prohibitions on signs almost always use ことはできません — with は rather than が — because a rule is contrastive by nature ("this, you may not do"):
ここでタバコを吸うことはできません。
koko de tabako o suu koto wa dekimasen
Smoking is not permitted here.
Now hear the same idea in casual speech, where the short potential wins effortlessly:
この子、もう自転車に乗れるんだよ。
kono ko, mō jitensha ni noreru n da yo
This kid can already ride a bike, you know.
Saying この子、もう自転車に乗ることができるんだよ to a friend is not wrong — it just sounds stiff, like reading from a manual. In a chat between friends, 乗れる is what a native reaches for every time.
Where ことができる earns its keep
Register aside, the analytic potential genuinely rescues situations the short potential handles clumsily:
It disambiguates the られる collision. For ru-verbs, the short potential 食べられる is identical to the passive and the honorific. When you need to be unmistakably clear that you mean ability, ことができる has no such overlap: 食べることができる can only be "is able to eat." This clarity is one reason writers favor it.
この施設では、犬も一緒に食事することができます。
kono shisetsu de wa, inu mo issho ni shokuji suru koto ga dekimasu
At this facility, you may dine together with your dog as well.
It reads more explicitly in long or formal sentences, where a bare short potential could be missed. In dense written Japanese — contracts, manuals, academic prose — 〜ことができる spells out the "is possible" so plainly that the reader cannot mistake it.
会員登録をすると、過去のすべての記事を読むことができます。
kaiin tōroku o suru to, kako no subete no kiji o yomu koto ga dekimasu
Once you register as a member, you can read all past articles.
Common mistakes
1. Double-marking the potential. The single most common error: stacking ことができる on top of a verb that is already in the potential form. Pick one route — either the short potential or the analytic frame, never both.
❌ 私は泳げることができる。
watashi wa oyogeru koto ga dekiru
Incorrect — 泳げる is already 'can swim'; don't add ことができる on top.
✅ 私は泳ぐことができる。
watashi wa oyogu koto ga dekiru
I can swim.
✅ 私は泳げる。
watashi wa oyogeru
I can swim.
The frame takes the dictionary form (泳ぐ), never the potential (泳げる). If the verb already means "can," it has no business inside ことができる.
2. Dropping こと. できる needs a noun (or nominalized event) in front of it. A bare verb cannot sit directly before が: you must either nominalize with こと, or use a genuine noun.
❌ 私は泳ぐができる。
watashi wa oyogu ga dekiru
Incorrect — a plain verb can't precede ができる; it needs the nominalizer こと.
✅ 私は泳ぐことができる。
watashi wa oyogu koto ga dekiru
I can swim.
✅ 私は水泳ができる。
watashi wa suiei ga dekiru
I can swim / I can do swimming.
3. Putting が on the object. This is the short-potential's が-object rule leaking where it doesn't belong. Inside ことができる the verb keeps its normal を.
❌ 私は日本語が話すことができる。
watashi wa nihongo ga hanasu koto ga dekiru
Incorrect — 話す keeps its を object; が belongs to こと.
✅ 私は日本語を話すことができる。
watashi wa nihongo o hanasu koto ga dekiru
I can speak Japanese.
4. Over-deploying ことができる in casual speech. Not ungrammatical, just wooden. If you say 週末は早く帰ることができる to a friend, you sound like a public-address announcement. Use the short potential.
❌ 週末は早く帰ることができるから、遊ぼうよ。
shūmatsu wa hayaku kaeru koto ga dekiru kara, asobō yo
Understandable, but stiff for a chat with a friend.
✅ 週末は早く帰れるから、遊ぼうよ。
shūmatsu wa hayaku kaereru kara, asobō yo
I can get home early this weekend, so let's hang out.
Key takeaways
- 〜ことができる = dictionary-form verb + ことができる, a second route to "can" alongside the short potential.
- Structurally it is nominalizer こと + が + できる — literally "the act of X is possible."
- The verb inside keeps its normal を object; the が belongs to こと, not to the object.
- The choice between 泳げる and 泳ぐことができる is register, not meaning: short potential for speech, analytic potential for signage, rules, and formal writing.
- ことができる also disambiguates the passive/potential/honorific overlap of られる — 食べることができる can only mean ability.
- Never double-mark (×泳げることができる) and never drop こと (×泳ぐができる).
Now practice Japanese
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Start learning Japanese→Related Topics
- The Potential Form: Expressing 'Can'N4 — An introduction to the potential form 可能形 — the stative conjugation that turns a verb into 'can / is able to,' and why its object leans toward が.
- Potential Formation by Verb ClassN4 — How to build the potential form class by class — られる for ru-verbs, the -eる shift for u-verbs, and the suppletive できる / 来られる irregulars.
- Potential + が for the ObjectN4 — Why the object of a potential verb usually re-marks from を to が, and the conditions under which を survives.