1、can能，可以，表说话人同意，许可还可表客观条件许可，如:You can go now.
提建议或请求时可用can I, can you表客气，如Can I buy you a drink?
can和be able to表能力时的区别。
can表一般具有的能力，be able to表在特定条件下的能力，如:Although the driver was badly hurt,he wasable to explain what had happened.
2、may
(1)可以，表说话人同意，许可或请求对方许可。You may go.
(2)(现在和将来)可能，也许，只用于肯定句和否定句中，如He may not be right.
3、must,have to
must表主观上的必须，have to表客观上的必须，如:It's getting late. I have to go. -Must I go now.-Yes,you must.(No, you needn't./ No, you don't have to.)
4、need,dare这二词有实意动词和情态动词两种词性，如用作实意动词后接动词不定式to do，如用作情态动词后接动词原形。Need I go now? --Yes, you must./No, you needn't.)
5、shall用于第一人称疑问句中表说话人征求对方的意见或向对方请求，如，Shall we begin our lesson?用于二、三人称陈述句，表说话人命令、警告、允诺、威胁等口吻，如: You shall fail if you don't workharder.
6、should表应该，意为有责任，有义务。如:We should try our best to make our country more beautiful.
7、will表有做某事的意志、意愿、安心、打算，如"Will you lend me your book?" "Yes, I will."
8、should have done表应该做而未做
must have done表对过去事实的肯定推测
could have done表本可以做某事
9、判断句：肯定句用must, 否定句用can't,不太肯定用may,might
He must be in the office now.
He must have gone to bed, for the light is out.
He can't be in the office. He is at home.
He couldn't have cleaned the classroom, because he didn't come here today.
He might be in the office, I am not sure.
He might have cleaned the room, I suppose.