could と was able to(過去の「できた」)Could vs. Was Able To: Two Kinds of "Could"
助動詞 / Modal Verbs
「昔は〜できた」という一般的な能力は could。でも「その場で1回やり遂げた」ことは was able to(または managed to)。肯定の I could escape とは言えないのに、否定の I couldn't escape は言える——この非対称が急所です。
For general past ability, could is fine: I could swim at five. But for pulling something off on one occasion, English wants was able to or managed to. The twist: affirmative could is off-limits there, yet couldn't works perfectly.
「できる状態だった」と「実際にやれた」
Having the Skill vs. Pulling It Off
could は「いつでもできる能力があった」という状態の話。1回きりの達成は「能力+実行」なので、実行まで含む was able to / managed to の出番です。否定になると「能力がなかった」も「その場でできなかった」も同じ結末に行き着くので、couldn't はどちらにも使えます。
Could describes a standing ability: you had the skill, any time. A one-time achievement is skill plus execution, so it calls for was able to or managed to, which include the doing. In the negative, "lacked the skill" and "failed that time" collapse into the same outcome — which is why couldn't covers both.
状態の could、達成の was able toStanding skill vs. one-time success
I could read before I started school.
学校に上がる前から字が読めた。
いつでもできた=一般的な能力。A standing ability — any time.
Theなぜ the なのか【その場で一つに決まる】話し手と聞き手のいる状況で、どれを指すか一つに決まるから the。日本語なら「（例の）」「うちの」を補って読むと感覚が近い。 fire spread fast, but everyone was able to escape.
火の回りは早かったが、全員が脱出できた。
その場で実際にやり遂げた1回。could は不可。Pulled off on one occasion — could won't work here.
I couldn't open theなぜ the なのか【その場で一つに決まる】話し手と聞き手のいる状況で、どれを指すか一つに決まるから the。日本語なら「（例の）」「うちの」を補って読むと感覚が近い。 jar.
瓶のふたが開けられなかった。
否定なら1回きりでも couldn't でOK。Negatives are safe: couldn't handles one-offs fine.
could
昔はいつでもできた(能力の状態)。You had the skill, standing by.
My grandfather could fix anything.
祖父は何でも直せる人だった。
was able to / managed to
そのとき実際にやれた(実行込み)。managed to は苦労の含み。Actually pulled it off that time. Managed to adds a whiff of struggle.
We managed to catch the last train.
なんとか終電に間に合った。
よくある間違い / Common Mistake
×I could pass the exam last week.
先週、試験に受かることができた。(のつもりで)
○I was able to pass the exam last week.
先週、試験に受かることができた。
1回きりの達成に肯定の could は使えません。could だと「受かる力はあった(実際に受けたかは不明)」という宙ぶらりんな響きになります。シンプルに I passed the exam. でも十分です。
Affirmative could can't report a one-time success. "I could pass" hangs in the air — you had the ability, but did you actually take it? Was able to closes the deal. Plain "I passed the exam" works too.
managed to ...なんとか〜できた「実行+ひと苦労」まで運ぶ。達成の報告の定番。Execution plus effort, in one word — the go-to for hard-won successes.
couldn't believe ...〜が信じられなかった否定の could は1回の場面でも自由。驚きの定番形。Negative could roams free — the classic shape of astonishment.
could hardly ...ほとんど〜できなかったhardly が実質否定なので could が使える。Hardly counts as a negative, so could is welcome.
could see / hear / smell見えた/聞こえた/においがした知覚や理解は「状態」扱いなので、1回の場面でも could が標準。Perceiving and understanding count as states, so could is standard even for single scenes.
まとめ / Summary
昔の一般的な能力 → could。General past ability → could.
1回きりの達成 → was able to / managed to(または普通の過去形)。One-time achievements → was able to, managed to, or just the simple past.
否定はどちらも couldn't でOK。In the negative, couldn't covers everything.
知覚・理解の動詞(see / hear / understand)は1回でも could。Verbs of perceiving and knowing take could even for single moments.