情态助动词表示“义务”和“必然'
1) 表示“义务”
所谓表示“义务' (Obligation), 即表示“应该'、“必须“做某事 ,可用 should,
should / ought to 表示“应该＂，带有表示“劝说”或“敦促＂的含义。
We should learn from the model workers.我们应该学劳模。
You should
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情态助动词表示“义务”和“必然'
1) 表示“义务”
所谓表示“义务' (Obligation), 即表示“应该'、“必须“做某事 ,可用 should,
should / ought to 表示“应该＂，带有表示“劝说”或“敦促＂的含义。
We should learn from the model workers.我们应该学劳模。
You should
do what your teachers tell you to.
He should do something to help her.
should /ought to 同义，都能表示“应该＂，往往可以交替使用。
You should /ought to drink less.你喝酒应该少点。
You should /
He shouldn't /
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 /
You should /
You shouldn't /
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和敦促时，往往用 should 而不用ought to。
You should mind your own business.
must (必须）表示“敦促＇或“命令”，在口气上比 should (表示“应该＂）更加强烈，这意味着说话人是权威的一方，对人对己都适用。
You must be back by ten o'clock.
I really must stop smoking. 我真的必须戒烟。
表示将来的“必须＂，常用半助动词 have to 的一定形式 (will /
We must do it again. (表示现在）我们必须做多一次。
We'll have to do it again. (表示将来）我们以后得再做一次。
表示过去的“必须＂，常用 have to 的过去时形式 had to。
I must leave at six. 我们必须离开的时间是六点。
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 /
You mustn't talk like that.
You are not to talk like that.
上述两句的过去时形式均为：
You weren't to talk like that.
must 的第二个否定形式是 need not /
A: Must you leave so soon? 我必须离开这么快吗？
B: No, I needn't.
needn't 相当的动词形式是 don't need to/ don't have to /
You must do it at once.
You needn't do it at once. 你不必做得这么快。
You don't need to do it at once. 你没有必要做得这么快。
You don't have to do it at once. 你不必做得这么快。
You haven't got to do it at once. 你不必做得这么快。
过去时形式是：
You didn't need to do it at once.你当时不必做得那么快。
You didn't have to do it at once.
表示“必须' , must 与
He must say it in English. (I want him to do so.)
He has to say
He must move the furniture himself. (I am too busy to help him.）
He has to move the furniture himself. (He's got no one to help him.)
最后，在特定语境中，用 will 也可表示“义务“。
You will wait outside the gate. (你必须等候在门外。）
You will report to me afterwards
will 的这一用法，常用于第二人称主语．这实际上是一种强化的祈使句，比带有第二人称主语的祈使句口
气更为强化。
Parent: Brian, close the door.
Brian: (No movement toward the door)
Parent: Brian! You close the door.
Brian: (Still no movement toward the door)
Parent: Brian! You will close the door!!!
表示“义务”，除用情态助动词外，还可用其他词汇手段。
In most countries, the law obliges parents to send their chiren to school.
Is attendance at school obligatory?
If, through carelessness, someone damages
He should do something to help her.
should /ought to 同义，都能表示“应该＂，往往可以交替使用。
You should /ought to drink less.你喝酒应该少点。
You should /
He shouldn't /
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 /
You should /
You shouldn't /
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和敦促时，往往用 should 而不用ought to。
You should mind your own business.
must (必须）表示“敦促＇或“命令”，在口气上比 should (表示“应该＂）更加强烈，这意味着说话人是权威的一方，对人对己都适用。
You must be back by ten o'clock.
I really must stop smoking. 我真的必须戒烟。
表示将来的“必须＂，常用半助动词 have to 的一定形式 (will /
We must do it again. (表示现在）我们必须做多一次。
We'll have to do it again. (表示将来）我们以后得再做一次。
表示过去的“必须＂，常用 have to 的过去时形式 had to。
I must leave at six. 我们必须离开的时间是六点。
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 /
You mustn't talk like that.
You are not to talk like that.
上述两句的过去时形式均为：
You weren't to talk like that.
must 的第二个否定形式是 need not /
A: Must you leave so soon? 我必须离开这么快吗？
B: No, I needn't.
needn't 相当的动词形式是 don't need to/ don't have to /
You must do it at once.
You needn't do it at once. 你不必做得这么快。
You don't need to do it at once. 你没有必要做得这么快。
You don't have to do it at once. 你不必做得这么快。
You haven't got to do it at once. 你不必做得这么快。
过去时形式是：
You didn't need to do it at once.你当时不必做得那么快。
You didn't have to do it at once.
表示“必须' , must 与
He must say it in English. (I want him to do so.)
He has to say
He must move the furniture himself. (I am too busy to help him.）
He has to move the furniture himself. (He's got no one to help him.)
最后，在特定语境中，用 will 也可表示“义务“。
You will wait outside the gate. (你必须等候在门外。）
You will report to me afterwards
will 的这一用法，常用于第二人称主语．这实际上是一种强化的祈使句，比带有第二人称主语的祈使句口
气更为强化。
Parent: Brian, close the door.
Brian: (No movement toward the door)
Parent: Brian! You close the door.
Brian: (Still no movement toward the door)
Parent: Brian! You will close the door!!!
表示“义务”，除用情态助动词外，还可用其他词汇手段。
In most countries, the law obliges parents to send their chiren to school.
Is attendance at school obligatory?
If, through carelessness, someone damages