1.can， could
表示能力、允许、客观可能性。
She is only four， but she can read.
Is there anything that I can do for you?
This sort of thing can't go on.
I couldn't follow her speech.
Could you lend me your bike?
Fire can't destroy gold.真金不怕火练
2.may， might
表示允许、猜测。
The dialect of one province may be quite different from that of the next one.
Students may not stay out after midnight without written permission.(stay out 在外边待的很晚)
He asked his mother if he might go out to play.
"May I open the window?" "Yes， please./ Please don't./ No， you mustn't." (may 如果要否定用mustn't，could和might用在现在时表委婉的说法)
3.must
表示"肯定"、"准是"，还可以表示"必须"，与have to 意思接近，但must 侧重说明主观看法，have to 强调客观需要。must时主观表达方式，have to是客观表达方式。mustn't 表示"千万不要"、"一定不要".
If you must go， at least you wait till the rain is over.
Teachers always tell us we must do everything step by step.
——Must I go tomorrow?
——No， you needn't.(Yes， I'm afraid so. must一般疑问句的否定用needn't来回答)、
He has to tidy up the room everyday.(tidy up 打扫干净)
(He must tidy up the room everyday.)
We have to study a foreign language.
You mustn't talk like that to your parents.
4.ought to ， should
表示应该做的事和非常可能的事。
You ought to go to see a doctor for you don't look well.
You shouldn't talk like that.
The old man said you ought to tell the police.
You oughtn't to smoke so much everyday.
The work ought to be finished by next Friday.
The book should be available in the bookstore. (available 可得到的)
5.need
表示"需要".need可以当情态动词，也可以做实义动词。
Need I come?-Yes， you must.(No， you needn't)
All you need do is to say yes to his questions.
He said he needn't hurry.
need 还可以作实义动词，后面接动名词时相当于被动结构。
You don't need to stay.(=You needn't say. )Does he need to know it?(= Need he know it?)
needs/ wants/ requires +doing相当于被动语态。
The question needs/ wants/ requires discussing.(=needs to be discussed.)
My shoes need repairing.(My shoes need to be repaired.)
物作主语时need接doing表被动。
6.dare
表示"敢"，也可以作实义动词，主要用于否定句、疑问句。
The boss is so hot-tempered that no one dare tell him the bad news.(so……that 如此……以至于)
Who dare stop you?
He doesn't dare to look out of the window.
情态动词没有人称性数的变化，有人称性数变化的一定是实义动词。
She dare not say what she thinks.(=She doesn't dare to say what she thinks.)
7.had better
表示"最好".否定为 had better not
I had better not disturb him.
We had better go and see the doctor now.
8.would rather
表示"宁愿"，后面接从句时，从句的谓语用一般过去时。否定用would rather not.
I'd rather not say anything.
John would rather work in a company than in a factory.
She would rather have the small oranges than the large ones.
I would rather you came tomorrow.
(would sooner 迟早，后的宾语从句用虚拟语气，一般过去时。)
I would sooner she left the heavy end of the work to someone else.
