必须（注意其肯定回答和否定回答）
I -must remind him of the truth,mustn't
- Yes,you must./ No,you needn't.
2 A:Everyone must work for a better tomorrow,mustn't they?
B:Yes,they must.
一定 (注意助动词的使用)
John must have seen the film,hasn't he?
John must have seen the film yesterday,didn't he?
John must have been to Japan before,hasn't he?
John must have been in Japan last week,wasn't he?
John must have left before Mary arrived,hadn't he?