can と be able to の違いCan and Be Able To: The Stand-In Rule
助動詞 / Modal Verbs
ふだんの「できる」は can で十分。be able to の出番は、can が文法的に形を作れないとき(未来 will be able to・現在完了 have been able to・to の後ろ)です。使い分けというより「can の代打」と覚えるのが実戦的。
For everyday ability, can does the job. Be able to steps in where can grammatically can't go — the future (will be able to), the perfect (have been able to), after to. Think of it less as a choice and more as can's stand-in.
can が座れない席がある
Seats Can Can't Sit In
can は助動詞なので、will can(未来)・to can(不定詞)・have canned(完了)という形が作れません。その空いた席に座るのが be able to です。意味の差はごくわずかで、be able to のほうが少し硬い。だから「席が空いたときだけ be able to」で、日常はほぼ困りません。
Can is a modal, so English simply has no "will can," "to can," or "have canned." Be able to takes those empty seats. The meaning barely shifts — be able to just sounds a touch more formal. So the working rule is: use can until grammar forces the stand-in.
代打が出る3つの席The three seats that need the stand-in
She can speak three languages.
彼女は3か国語を話せる。
ふだんの「できる」は can。Everyday ability — plain can.
You'll be able to see Mt. Fuji from theなぜ the なのか【決まった場所・乗り物】the station / the bus のように、話し手と聞き手の生活で「あの一つ」に決まる施設や便を指す。 train.
電車から富士山が見られますよ。
will の後ろは can が入れない → be able to。After will, can can't follow — the stand-in steps in.
I want to be able to swim.
泳げるようになりたい。
to の後ろも can 不可。After to, same story.
can
現在の能力・可能。日常の標準。Present ability. The everyday default.
Can you drive?
運転できる?
be able to
can が入れない席(未来・完了・to の後)。やや硬い。The seats can can't take — future, perfect, after to. Slightly formal.
I haven't been able to sleep well lately.
最近よく眠れていない。
よくある間違い / Common Mistake
×I will can drive next year.
来年には運転できるようになる。(のつもりで)
○I'll be able to drive next year.
来年には運転できるようになる。
標準英語では助動詞を2つ重ねられません(will can は不可)。未来の「できる」は will be able to。これがこのペアの一番大事なルールです。なお米南部やスコットランドの方言には might could(できるかもしれない)のような二重助動詞が実在しますが、標準の書き言葉では使いません。
Standard English doesn't stack two modals — "will can" doesn't exist. Future ability is will be able to, and that's the single most important rule in this pair. Double modals like might could are real in Southern US and Scottish English, but standard writing steers clear of them.
can afford〜する(金銭的・時間的)余裕があるafford は can とセットで使うのがほぼ決まり。can't afford(余裕がない)が特に頻出。Afford practically lives with can — especially can't afford.
can't help -ing〜せずにいられない「(笑うのを)止める能力がない」という理屈から。I can't help laughing.Literally "lacking the power to stop it": I can't help laughing.
as soon as I canできるだけ早く「自分の能力の限界いっぱいまで」の can。as ... as I can 型の仲間が多い。Pushed to the edge of your ability — one of many as … as I can patterns.
be able to make it都合がつく・間に合うmake it(たどり着く・都合をつける)と組む定番。Will you be able to make it?Pairs with make it: Will you be able to make it?
まとめ / Summary
ふだんは can。be able to は「can の代打」。Default to can; be able to is the stand-in.
未来 will be able to・完了 have been able to・to の後ろ、が代打の席。The stand-in's seats: after will, in the perfect, after to.
標準英語では助動詞を重ねられない(× will can)。Standard English doesn't stack modals — no "will can."
Will you be able to ...? は丁寧な打診にもなる。Will you be able to …? doubles as a gentle inquiry.