couldn't和can't的区别：在请求语句里，couldn't比can't较委婉。在一般语句里，couldn't表过去时，can't表现在时。例句：I couldn't tell if he had been in a fight or had just fallen down.
couldn't和can't的区别
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”。
can't造句
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.