may                permission

can                 posibility

           ability

may: 用于长辈与晚辈之间，上下级之间，不是很的熟

could: 客气的

can: 非常熟，同辈之间，很随便的

1. permission

asking for permission:

Could I use your bike?

Of course, you can.（如用could回答,意为"你用吧"，有犹豫的不想借之意）

转达用can,不能用may

It's not fair, Mary can stay up till ten and I have to go to bed at eight.

              may:现在的，即将的情况推测，判断

2. posibility 

can: 抽象的，范范的谈

Will you answer the phone?

a.

It may be your mother.

According to the radio it may rain this evening.

Guangzhou can be very hot in summer.

Mr. Smith looks pale. He may be ill.

Mr. Smith is in poor health. He can be ill at any time.

Where is John?

He may be in the library.

Other modal verbs for deduction:

He must/will/would/ought to/should/could/may/might be in the library.

b.

He may 'not go tomorrow. (= I do not permit him to go)

He 'may not go tomorrow.(= It is possible that he will not go)

Interrogative: 疑问句用can和could

( It is midnight, there is a knock at the door)

Who can/could that be, so late?

It may/…(就是不能用can，其他都可用) be Tom, without bringing the key againg.

Negation: could't（不大可能）

It may not be Tom. （may not可能不）

It can't be Tom. He has gone to Beijing.（can't不可能，）

3. ability（其中有现在，将来，过去的ability）

1) present and future ability: can/be able to present and future

a. present ability

Can you/Are you able to type?（用Can you意为"懂的"，用Are you able to意为"不是很懂"）

b. future ability（纯粹能力）

Our baby will be able to walk in a few weeks.（纯粹将来的能力）

c. present arrangement for future action（现在对未来的安排）

Can you come to a party on Saturday evening?（常用）

Will you be able to come?（费力点，不常用）

2) past ability: could OR was able to

a.When I was young I          climb any tree in the forest.

Although the pilot was badly hurt, he was able to explain what had happen.

He read the message but he          (not) understand it.

I          see him through the window.

（一般性的能力用could 和be able to都可以）

b. for particular occasion: 过去具体某事用was able to

Although the pilot was badly hurt he was able to explain what had happen.

I knew the town so I was able to advise him where to go.

The car plunged into the river. The driver was able to get out but the passengers were drowned.

（表否定和感观动词无以上区别）

c. exceptions

He read the message but he couldn't/wasn't able to understand it.

I had no key so I couldn't/wasn't able to lock the door. (negation.)

I could/was able to see him through the window. (verbs of senses)

3) could+perfect infinitive:

a. ability

He was able to send a message. ( He did it)

He could have sent a message.( He didn't sent it OR we don't他本来可以，但没有做)

b. deduction

The money was disappeared; who could have taken it?

Tom could have (taken it); he was alone here yesterday.

He can't/couldn't have taken it; he was away all day.

must/have to/ought to/should

must表示说话人的权威(决定权),用于上下级、长辈与晚辈之间

You must obey Mr. Pitt.

You must attend the meeting.

You have to attend the meeting. ( have to 指外界的原因,在说话人与受话人以外的原因)

You must wash your own clothes.

You have to wash your own chothes at school.(外界原因)

ought to/ should都表责任和义务，应该…

1.pianist to pupil: you must practice at least an hour a day

2.pupil to a musical friend: have to practice at least an hour a day

3.musical friend: you ought to/should practise for more than an hour

（紧急重要的，表主动的，自己本人的决心用must；表无可奈何，外界的用have to）

children must obey their parents. （must是说话人的意见、主张）

children have to obey their parents. （have to 不反映说话人的意见、主张，是外界的。） 

第一人称

I must/ will/ have to buy a dictionary.

I must/ have to/ will have to cut down on smoking.

I have to take two of these pills a day.

I have to is better for habits.

I must be at the station at ten. It's most important/ urgent.

I must tell you about a dream I had last night.(important)

ought to含有责备的含义，而should没有，发通知用should

Cadidates should be prepared to answer question on…

On hearing the alarm bell, hotel guests should leave their room.

have to             don't have to

（习惯性、一次性动作均可）   don't need to (need是实意动词)

否定：must    needn't（说话人的权威），need是情态动词

You must come this afternoon.

You needn't come this afternoon. (boss告知员工，表现其权威)

a. 现在时: have got to用于一次性的的行为 (needn't     don't' have to     haven't got to)  

You needn't call me Mr. Jones. We all use first name here. (没说话者权威)

We don't have to type our essays but we don't have to / haven't got to write legibly.

Ann hasn't got to go to this lecture. Attendance is optional.

b. 用于过去的（过去的事情已不存在说话者的权威，只是一种述说）

I didn't have to / didn't need to / hadn't got to wait long. He was only a few minutes late. 

long用于否定句，for a long time用于肯定句

When he was at university he didn't have to / didn't need to pay anything for his keep, for he stayed with his uncle. (习惯动作)

Must I go. Mother? (表猜测)

Need I go. Mother? (表不想去的提问，希望得到否定回答)

A: Need he report the audient? (希望得到否定回答)

B: Of course he's got to. (具体某次)

A: Well, does he need to report it straight away. (实意动词，不会否定意义.straight away =at once)

A: Need we work late today? （need通常和must搭配用）

B: No, but we must tomorrow.

4. needn't have+过去分词      didn't have to 

（不定式的完成时，站在现在角度对过去的讨论，表本来不用做，但已做了动作）

You needn't ha