英語例文 英文法

can/couldとbe able toの使い分け

● 現在や未来の能力について話す場合と、過去の全般的な能力について話す場合は、can/could と be able to を同じ意味で使える。

She can speak Spanish very well.	＼ ／	この2つの文は同じ意味
She is able to speak Spanish very well.

I could run much faster when I was younger.	＼ ／	この2つの文は同じ意味
I was able to run much faster when I was younger.

※ ただし、以下のように現在完了時制の場合や ...ing 形にする場合、can/could は使えないので、be able to を使う必要がある。

He has been able to swim since he was two years old.
I like being able to choose my own working hours.

● 過去の特定の時点における特定の能力について話す場合、could を使うことはできない。

The traffic was really bad, but we could catch our train. ← この文は間違い
                                                ↓
The traffic was really bad, but we were able to catch our train.
The traffic was really bad, but we managed to catch our train.

※ ただし、否定形の場合は、内容が全般的か特定的かにかかわらず could を使える。

It was hard work, and we couldn't finish everything on time.	＼ ／	どちらもOK
It was hard work, and we weren't able to finish everything on time.

● 仮定の意味で could が使われることがある (「仮定法過去」を参照)。

I couldn't help you because I had too much to do.
                           ↓
これは過去の意味なので、次のように書き換えられる。
                           ↓
I wasn't able to help you because I had too much to do.

I couldn't help you even if I wanted to.
                           ↓
これは仮定の意味なので、次のように書き換えられる。
                           ↓
I wouldn't be able to help you even if I wanted to.

※ その他の例文
Why couldn't he do it last week. (過去)
We could work together, if you wanted. (仮定)