a. You ought to obey your teachers.
表示说话者要听话者注意自己有服从教师的义务，不含强迫之意。
b. You must obey your teachers.
表示说话者要听话者必须服从教师，含有主观强制之意。
c. You have to obey your teachers.
表示按照规章制度，听话者有服从教师的义务，含有外界强制之意。（客观）
2、must和have to在过去时句中的用法:
She had to leave at six yesterday afternoon.
她昨天下午六点必须离开。
He said that he must leave at five that afternoon.
他说他那天下午五点必须离开。
3、must
A: Must I go?
肯定回答：B: Yes, you must/ have to /have got to.
否定回答：No, you needn't.(一般不用 mustn't)
No, you don't need to.
No, you don't have to.
Why must you always bother me?
为什么你总是来烦我?
She isn't here. She must be in the library.
她不在这儿。她一定在图书馆里。
She must be watching TV at home now.
她现在一定在家看电视。
You must have left your umbrella in the theater yesterday.
你昨天肯定把自己的伞忘在剧场里了。
She can't have been to your home.She doesn't know your address.
她不可能去过你家。她不知道你的地址。
4、ought to:
You ought to be hungry now.
你现在大概饿了吧。
This oughtn't to cause any problem.
这大概不会引起什么麻烦吧。
She ought to be here very soon.
她可能很快就到这儿来。
You ought to have come earlier.
你本应该早来一点嘛。
You oughtn't to have turned down her kind invitation.
你不应该拒绝她的盛情邀请。
Ought you smoke so much?
你应该抽这么多烟吗?
You oughtn't smoke so much.
你不该抽这么多烟。