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.

can't和couldn't表示否定的推断

对现在事件的否定推断，可以用can't或couldn't加上动词be的现在式或者可以用can't／couldn't加上动词的进行式表示：

Child：Can I have some sweets？I'm hungry．

孩子：我可以吃些糖果吗？我饿了。

Mother：You can't／couldn't be hungry. You've just had dinner．

母亲：你不会饿的。你刚吃过饭。

can't例句

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。

couldn't例句

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。