二、may, might

1. 表示询问许可或允许
—May/Might I use your computer? —Of course you can/Sorry, I’m afraid you can’t.
2. 表示猜测，“可能、或许”。might可以表示过去、现在、将来的可能性（可能性较小）。may不能用于疑问句表可能性。
She may not call you back.
We may move abroad to live next year.
3. may/ might have done用于对已经发生过的情况的肯定推测判断，语气比较弱，意思是“可能，或许…”。例如：
She didn’t get there in time. She may have missed the bus.
He didn’t come to school today. He might have been ill.　
They may have been separated from the other students and…
4. might have done还可表示对过去可能发生而实际上没有发生的事情。
You drove too fast. You might have killed yourself.
5. may well +动词原形“（完全）能,很可能”。 may as well/might as well+动词原形:“最好…,倒不如…”=had better。
She has changed so much that you may well not recognize her.
You may as well do it in another way.
但I might as well be dead, for all you know. (即使我或许会死去，你也并不在意。本句中for all sb.knows/cares主要用于口语，意思是“（即使...）某人并不知道/在意”。口语就是这样，习以为常就好了。)

三、must与have to

1. must表示“必须”，强调说话人的看法，可表示过去、写作或将来，have to强调客观需要，有不同的时态、语态。must表示现在、将来的“义务”； 表示过去的义务用have to
2. mustn’t表示“禁止”，needn’t、don’t need to、 don’t have to表示“没必要”。回答must引起的问句时，否定回答用don’t need to、 don’t have to．
3. must可表示“偏偏”之意，指令人不悦。
The car must break down when I was about to start.
John, look at the time. Must you play the piano at such a late hour?
4. must还可表示必然性。
If you don’t hurry, you must miss the first bus.
5. 表示推测，“肯定是、准是”。“must+动词原形”表示对现在的推测；must be doing sth.表示对现在和将来正在的推测; must have done表示对过去的推测。