情态动词的使用：can, must, may, might, need,
1.情态动词can(能够), must(必须), may(可以)
结构：主语+can/must/may+动词原型
He can make the tea.
Sally can air the room.
We can speak English.
变疑问句将情态动词移到句首
Can he make the tea?
Can Sally air the room?
Can we speak English?
变否定句在情态动词后面加not
He cannot make the tea.
Sally cannot air the room.
We cannot speak English.
肯定回答及否定回答
Yes, he can. No, he cannot.
Yes, she can. No, she cannot.
Yes, we can. No, we cannot.
特殊疑问句：
What can you do?
注意：情态动词的句子没有第三人称单数的变化，不要在情态动词或动词后面加S。
2.Must/have to的区别
must 表示必须，是主观上觉得应该做，have to是不得不，是由于客观条件逼迫的必要要做
must 只能用在表示现在和将来的句子里，而have to do可以用在任何时态
3.must, may, might表示猜测：
must do 表示对现在事实的猜测
must have done表示对过去事实的猜测
must have been doing 表示对过去正在进行的事实的猜测
may/might do, may/might have done表示没有任何事实依据的猜测，might的可能性更小。
can’t/couldn’t 表示不可能
4.need 用法：
表示“需要”时为实意动词，后面可以加名词，也可以加不定式：
I need a pen. Do you need any beer? No, I don’t.
I need to have a rest.
Need doing=need to be done，表示被动
The flowers need watering.
Need在否定时做情态动词使用
You needn’t go so early. =You don’t need to go so early.
Must I clean the desk right now? No, you needn’t.