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。
