違い、説明できますか？ 何となく使い分けているけれど明確な違いを説明できない、そんな方は結構多いと思います。難しくないので是非マスターしましょう！
“can”
「現在の一般的な能力」を表す場合と、「実現性の高い可能性」を表す場合の２種類あるので、それぞれ詳しく見ていきます。
現在の一般的な能力
「現在の一般的な能力」は、現在において一般的な（一時的ではなく継続した）能力を持つことを表すのに使います。
- She can swim better than him.
- He can draw portraits even in the absence of the subjects.
- You can enjoy a great night view from your room.
- Politicians can remember only what they want to.
- It can be said that taking a good sleep will improve your immune system.
- In block chain systems, no changes can be made to data which have gotten written in a block.
実現性の高い可能性
「実現性の高い可能性」は、できごとの起こる可能性が高いことを表すのに使います。
- It can be hard to have a lion as a pet. It’s not like having a cat.
- There can be many people in the amusement park in weekends.
“could”
「過去の一般的な能力（”can”の過去形）」を表す場合と、「実現性の低い可能性」を表す場合の２種類あるので、それぞれ詳しく見ていきます。
過去の一般的な能力（”can”の過去形）
「過去の一般的な能力（”can”の過去形）」は、過去において一般的な（一時的ではなく継続した）能力を持つことを表すのに使います。
- She could speak 3 languages.
- He could tell a good wine from a bad one.
- He recovered from a severe pain in his back and could move.
- He was so rich that he could buy whatever he wanted.
- There used to be less skyscrapers, and you could see the mountain in the distance.
- The factory polluted the river to the extent in which even black basses could not survive.
実現性の低い可能性
「実現性の低い可能性」は、できごとの起こる可能性が低いことを表すのに使います。
- I know you have a puma. But it could be hard to have a lion as a pet.
- There could be many people in the amusement park even in weekdays.
“be able to”
「”can”や”could”の文法的制約」に使われる場合と、「一時の達成・成果」を表す場合の２種類あるので、それぞれ詳しく見ていきます。
“can”や”could”の文法的制約
「”can”や”could”の文法的制約」は、“can”や”could”が文法の制約上使えない場合に使います。
- I haven’t been able to sleep lately.(You cannot say “I haven’t can…”)
- She will be able to swim well after some practice.(You cannot say “I will can…”)
一時の達成・成果
「一時の達成・成果」は、一時的な達成・成果（一般的な能力への言及ではないこと）を表すのに使います。
- She was able to swim across the river despite the fierce current.
- The team was finally able to win a championship.
- He was able to find her because she was just like her photo.
- The forest fire spread to their home, but they were able to escape.
- The thief took advantage of the weak security and was able to steal paintings from the museum.
- The demonstration was able to draw the support of people suffering economically.
なお、「一時の達成・成果」であっても、否定文や特定のフレーズ（”all S can/could do”や”the only thing S can/could do”等）では、”can”や”could”を使う方が一般的です。恐らく、”be able to”だとリズムが悪いからだと思います。
- He could not find her because she was totally different from her photo.
- He got a severe pain in his back and could not move.
- The only thing the government can do during the pandemic is (to) distribute to people masks of poor quality.
- All I could do to recover from the loss Bitcoin had brought on me was (to) sell my belongings.