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.

相关例句

can't

1、I can't eat any more; I'm full up.

我不能再吃了，我已经很饱了。

2、I don't want to leave, but I can't go on.

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。

4、Don't tell me you can't remember.

不要告诉我你记不起来了。

couldn't

1、Jamie e-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 imagine why he would want to be alone with me

我想象不出为什么他会想和我独处。

3、They put a screen in front of me so I couldn't see what was going on.

他们在我面前竖了一面屏风，所以我看不到发生了什么事情。

4、I couldn't get back to sleep.

我再也睡不着了。