can't/couldn't表示推测时，意思是“不可能...”，语气非常坚决的否定推测。
根据动作发生的时间，一般分为以下三种情况：
1）进行式（对正在发生的事情的否定推测）：can't be doing sth
如：It's already ten o'clock. My father can't be sleeping now.
2）完成式（对过去的动作的否定推测）：can't/couldn't have done sth
如：He is at schoool, so he can't/couldn't have stayed up last night.
3）一般式（对现在/将来情况的否定推测）：can't do sth
如：Your answer can't be true.
can/could表示推测时一般只用于否定句/疑问句，但是也可用于肯定句，只是比较少见。
其中could不表示过去式，而表示可能性较小。
拓展资料
can't
1、I'm afraid I can't remember the way
我恐怕不记得路了。
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。
5、I can't eat any more; I'm full up.
我不能再吃了，我已经很饱了。
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 tell if he had been in a fight 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 be alone with me
我想象不出为什么他会想和我独处。
4、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。
5、I couldn't get back to sleep.
我再也睡不着了。
本回答被网友采纳在请求语句里面couldn't比can't较委婉
在一般语句里面couldn't表过去时can't表现在时
can't
1、I'm afraid I can't remember the way
我恐怕不记得路了。
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。
5、I can't eat any more; I'm full up.
我不能再吃了，我已经很饱了。
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 tell if he had been in a fight 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 be alone with me
我想象不出为什么他会想和我独处。
4、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。
5、I couldn't get back to sleep.
我再也睡不着了。