学习重点：Modals:must 情态动词must
Modals: can 情态动词can
Modals: need 情态动词need
Modals: must 情态动词must
我们已经尝过了表示义务的must和have to。在现在时里面可能使用must和have to，但在其他时态里只能have to。
Postive Obligation 表示肯定意思的和义务（时态）
现在 主语+must+动词*
主语＋have to +动词
主语+have got to +动词
将来 主语+will+have to +动词
主语+be going to =have to +动词
We will have to see
He is going to have to drive
过去 主语+had to +动词
You had to resign.
现在完成 主语+have tp +动词
I have had to stop
She has had to sing.
Negative Obligation 表示否定意思的责任和义务
表示否定的义务，即禁止时，在现在时里可以用must,而在其他时态里则必须用be forbidden to 或not be allowed等。
现在 主语+must not(mustn’t )+动词
You mustn’t leave.
将来 主语+will be forbidden to+动词
主语+be going to be forbidden to +动词
主语+will not be allowed to +动词
主语+be not going to be allowed to +动词
You will be forbidden to leve.
You are going to be forbidden to leave.
He will not be allowed to eat.
He’s not going to be allowed to eat.
过去 主语+was/were forbidden to +动词
主语+was /were not allowed+动词
I was forbidden to speak
We were not allowed to see.
现在完成 主语+have been forbidden to+动词
主语+have not been allowed to +动词
He has beren forbidden to move.
They have not been allowed to talk.
No Obligation不做……也行
现在 主语+do not have to +动词
主语+have not got to +动词
I don’t have to go.
She hasn’t got to look.
将来 主语+will not（won’t)+have to +动词
主语+be not going to =have to +动词
I won’t have to lie.
We aren’t going to have to move.
过去 主语+did not have to+动词
She didn’t have to dance.
现在完成 主语+have not had to +动词
I haven’t had to work.
*此处指动词原型，也就是不加to的不定式。
Modals: can 情态动can
Expressing permission表示许可
现在时里可以用can, 过去时和条件时态里要用could,而其他时态里必须用be allowed to ,be permitted to 等表达。
现在：主语+can+动词
主语+be+allowed/permitted +to+动词
I can go .
She’s allowed/permitted to leave.
过去：主语+could动词
主语+was/were +allowed/permitted +to +动词
We could see.
It was permitted/allowed to continue.
将来：主语+will be allowed to +动词
主语+be+going to be allowed/permitted to +动词
I will be allowed/permitted to travel.
We are going to be permitted/allowed to attend.
条件：主语+could+动词
主语+would be +allowed/permitted +to +动词
He could do it.
We would be allowed/permitted to visit.
Expressing Ability 表示能力
现在时态里可以用can,过去时态和条件时态里要用could, 而其他时态里必须用be able to.
现在：主语+can +动词
主语+be able to +动词
I can dance.
We are able to sing.
过去：主语+could +动词
主语+was/were+able to+动词
You could draw.
They were able to choose.
将来：主语+will be able to +动词
主语+be going to be able to+动词
We will be able to fly.
She is going able to travel.
条件；主语+could+动词
主语+would be able to +动词]
They could drive.
He would be able to work.
Expressing possibility 表示可能性
在现在时态里用can,过去时里用could。
The weather can be very cold at this time fo year.每年这个时候都很冷。In the old days, the people of bognor could be very aggressive. 在古代，伯格诺人是非常好斗的。
Modals:need 情态动词need
Need后面加不定式，可以变成表示“有必要”的情态动词“。
在现时态里，它和一般动词一样，接在单数第三人称后面，要加-s;在否定句、疑问句里要用do/does.
*19\20单元里，为了方便起见，我们把意思上类似情态动词的表达（have need to ）称作情态动词。
●肯定句 I need to see you.我应该去见你。
He needs to see you.他应该去你。
●疑问句 Do you need to come? 你一定要来吗？
Does he need to come?他一定要来吗？
●否定句：You don’t need to come.你不一定要来。
He doesn’t need to come. 他不一定要来。
或者： You needn’t come.
He needn’t come.
即使在单数第三人称后，needn’t 也不参加-s,而且它后面接的是不加to的不定式。
它的过去时和将来时的方法和一般动词一样。
They needed to talk to you.
Did rhey need to talk to you?
They didn’t need to talk to you.
They will need to talk to you.
They are going to need to talk to you.
Will they need to talk to you?
Are they going to need to talk to you?
They won’t need to talk to you.
They ‘re not going to need to talk to you.
英国英语里还有needn’t have 这种过[的表达方法，表示的是“虽然没必要做，但还是做了”。
You needn’t have told him the truth.你不应该告诉他实情。
