1：表示“义务”
所谓“义务”，即表示“应该”、“必须”做某事，可用should ,ought to ,must .用这三个情态动词表示“义务”，其口气强弱略有区别。
用should /ought to 表示“应该”，带有表示“劝说”或“敦促”的含义。
eg:
We should learn from the model workers .
You should do what your teachers tell you to .
He should do something to help her .
should 与ought to 同义，都能表示应该，往往可以交替使用。
eg:
You should /ought to drink less .
You should /ought to tell the police .
He should n't/oughtn't to stay up so late .
用should口气比较肯定一些，说话人的主观信心要强烈一些，而用ought to 则在口气上不是那么坚定。
should /ought to 不定式完成时，可表示本来应该做某事却没有做或本来不该做某事却做了。
eg:
You should /ought to have asked my permission first .
You shouldn't /oughtn't to have been resting at that time of day .
正因为should 在口气上带有更多的主观信心，从而在表示强烈的劝说或敦促时，往往用should 而不用 ought to .
eg:
You should mind your own business .
用must （必须）表示“敦促”或“命令”，在口气上比should 更加强烈，这意味着说话人是权威的一方，对人对己都适用。
eg:
You must be back by ten o'clock .(你必须十点以前回来）
I really must stop smoking .(我真得必须戒烟了）
表示将来的“必须”，常用半助动词have to 的一定形式（will/shall have to ）.
eg:
We must do it again .(表示现在）
We'll have to do it again.(表示将来）
表示过去的“必须”，常用have to 的过去时形式 had to .
eg:
I must leave at six.
I had to leave at six yesterday.
must 有两个否定形式：第一个否定形式是 must not /mustn't ,表示“不许”、“禁止”，相应的半助动词是 be to 的否定形式 be not to 。
eg:
You mustn't talk like that .
You are not to talk like that .(不许你如此讲话）
上述两句的过去时均为：
You weren't to talk like that .
must 的第二个否定形式是 need not /needn't ，表示“不必”。
eg:
A: Must you leave so soon ?
B:No ,i needn't
与needn't 相当的动词形式是 don't need to /don't have to /haven't got to .
eg:
You must do it at once .
You needn't do it at once .
You don't need to it at once .
You don't have to do it at once .
You haven't got to do it at once .
过去时形式通常是：
You didn't need to do it at once .
You didn't have to do it at once .
表示“义务”，除用情态助动词外，还可用其他词汇手段。
eg:
In most countries ,the law obliges parents to send their children to school.
Is attendance at school obligatory ?
He was compelled by illness to give up his studies .
A defeated enemy usually signs a treaty under compulsion.
Military service is compulsory in many countries .
