shall 用于第一、第三人称疑问句中，表示说话人征求对方的意见 What shall we do next?

Shall I do the washing-up?

When shall my brother be able to leave hospital? 用于第二、第三人称的陈述句中，表示说话人给对方的命令、警告、允诺或威胁 You shall go with me. ( 命令)

You shall have a new dress for your birthday.（允诺）

He shall be punished.（威胁）

should 表示劝告或建议，作"应该"讲 We should be strict with ourselves.

You should keep your promise. 表示推测，作"可能"、"该"讲 The roads should be less crowded today.

I should have finished reading it by Friday. Why / how + should结构表示说话人对谋事不能理解、感到惊异等意思，译为"竟会" Why should you be so late today?

（你今天怎么来的这么晚？）

I don't know why you should think that I did it.

（我真不明白你凭什么认为这件事是我干的。） 5. must, may (might) 和can表示"推测"的用法

情态

动词 用法 例句 must 只能用于肯定句，表示可能性很大的一种推测，作"肯定""一定"讲 (1)--- Look, someone is coming. Who can it be?

--- It may be the headmaster.

---It can't be him. He has gone to Shanghai.

--- Then it must be Mr. Zhang. He looks like the headmaster.

(2)--- What can they be talking about?

----They may / must be talking about the question raised at the meeting.

(3)--- Can they have finished the work?

--- Yes, they may / must have finished it.

--- No, they can't have finished the work so soon.

--- Then they must have played so much. may /might 用于肯定句中（可能，或许，大概）；用于否定句中（可能不）。表示可能性较小的一种推测 can 表示推测时，只用于否定句和疑问句中 6.should与ought to用法

对比点 should ought to 1．表"应该" 表劝告、建议

You should listen to the doctor's advice.

We should learn from Zhang Hua. "有责任有必要"做某事

You ought to finish your work before you go home.

We ought to help each other. 2．表"估计"

They should get home by now. "非常可能"的事，可译为"总应该"

If we start to work right now, we ought to finish it before lunch.