1. must 和 can't 表示推测时确定程度最高，但是前者表示对事实的肯定推测，而后者表示对事实的否定推测； may 和 might 表示推测时确定程度均不高，虽然前者比后者要稍高一些，两者既可以表示对事实的肯定推测，也可以表示对事实的否定推测。例如：
- He must be in his room now.
- He can't be in the library now.
- I'm not sure where he is. He may/might be in the library.
- I don't know whether he is there. He may/might not be there.
- He must be Chinese.
- He can't be doing his homework now. He must be watching TV.
- He must have finished his homework.
- He can't have been doing his homework yesterday afternoon.