在請(qǐng)求語(yǔ)句里，couldn't比can't較委婉。在一般語(yǔ)句里，couldn't表過(guò)去時(shí)，can't表現(xiàn)在時(shí)。例句：I couldn't tell if he had been in a fight or had just fallen down.

can't和couldn't表示否定的推斷

對(duì)現(xiàn)在事件的否定推斷，可以用can't或couldn't加上動(dòng)詞be的現(xiàn)在式或者可以用can't／couldn't加上動(dòng)詞的進(jìn)行式表示：

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

孩子：我可以吃些糖果嗎？我餓了。

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

母親：你不會(huì)餓的。你剛吃過(guò)飯。

can't例句

1、I can't eat any more; I'm full up.

我不能再吃了，我已經(jīng)很飽了。

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

我不想離開(kāi)，但我繼續(xù)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

杰米給我發(fā)了電子郵件，說(shuō)他不能來(lái)了。

2、I couldn't imagine why he would want to be alone with me

我想象不出為什么他會(huì)想和我獨(dú)處。

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

他們?cè)谖颐媲柏Q了一面屏風(fēng)，所以我看不到發(fā)生了什么事情。