Explanation
Use "could" to talk about an ability in the past. It is the past form of "can."
For example, "I could swim when I was five." Here "I" is the subject, "could" shows past ability, and "swim" is the base verb. "When I was five" tells us the time in the past. In Japanese, you often say "5歳のとき泳げた". English shows the time in the modal sentence itself, so use "could" with a past time word.
過去にできたことを言うときは "could" を使います。これは "can" の過去形です。例えば "I could swim when I was five." では、"I" が主語、"could" が「〜できた」を表し、"swim" は動詞の原形です。"when I was five" は「5歳のとき」という過去の時を表します。日本語では「5歳のとき泳げた」と言いますが、英語ではこの形の中で過去の能力を表します。
Examples
- I could swim when I was five. 私は5歳のとき泳げました。
- She could read very well as a child. 彼女は子どものころ、よく読めました。
- Could you ride a bike when you were young? 若いころ、自転車に乗れましたか。
- He couldn't open the jar, so I helped him. 彼はそのふたを開けられなかったので、私が手伝いました。
- We could see the mountain from our hotel. 私たちはホテルからその山が見えました。
- I couldn't sleep last night. 昨夜は眠れませんでした。
Common Mistakes
Japanese learners often use "can" with a past-time word, like "I can swim when I was five." That is wrong. Use "could" for past ability: "I could swim when I was five." Also, do not add "to" after "could": say "could swim," not "could to swim."
日本人学習者は、過去の時間を入れても "can" のままにしてしまいがちです。たとえば "I can swim when I was five." は誤りです。過去の能力は "could" を使って "I could swim when I was five." と言います。また "could to swim" のように "to" を入れないようにします。