1、表示时间不同:couldn't代表的是过去的时间,表示以前某种情况不能发生或者某种行动无法完成。例如:“I couldn't finish my homework last night”。而can't则代表的是现在或者将来的时间。它指的是当前或者未来某种事情是不可能发生的。例如:“I can't play basketball”。
2、语气方面不同:在请求语句里,couldn't比can't较委婉。例如:“I couldn't disagree with you more”。
1、You can't measure it.
2、You can't edit what you can't see.
3、I can't keep quiet.
4、I can't run faster.
5、We can't forgive his impoliteness.