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:在请求语句里,couldn't比can't较委婉。例如:“I couldn't disagree with you more”。
couldn't:
1、Jamiee-mailed me to say he couldn't come
杰米给我发了电子邮件,说他不能来了。
2、I couldn't tell if he had been in afight or had just fallen down
我不知道他是和人家打架了,还是仅仅摔了一跤。
3、I couldn't imagine why he would want to bealone with me
我想象不出为什么他会想和我独处。
can't:
1、I'm afraidI can't remember the way
我恐怕不记得路了。
2、I don't want to leave,but I can't goon.
我不想离开,但我继续不下去了。
3、I can't drink any more.
我不能再喝了。