have to

have to加动词原形

表示"不得不"、"必须"的概念。它比must更含有"客观条件使得必须如此做"的意思，并有较多的时态。如：

Mr. Johnson has to work very hard to earn a living.约翰逊先生为了谋生不得不拼命干活。

I had to leave the party early last night. I wasn’t very well.昨晚我只得早点离开晚会，我不大舒服。

We will have to get up very early tomorrow.我们明天必须起得很早。

I shall have to go to the clinic today for my bad cough.我咳嗽得厉害，今天必须去诊疗所看看。

[注]口语中的I have got to，you have got to等：I have to，you have to. Have I got to?和Do I have to?这两种疑问形式均可用。

shall

shall作为情态动词用于第二、三人称

可表示说话人给对方的"命令"、"警告"、"允诺"、"威胁"等概念。如：

You shall do it，whether you want to or not. 不管你愿意不愿意，你必须做这项工作。(命令)

You shall have the book as soon as I finish it.这本书我一看完就给你。(允诺)

You shall pay for it.你一定会受到惩罚的。(威胁)

在疑问句中，情态动词shall用于第一、三人称

表示说话人征求对方的意见或向对方请示。如：

What shall I do now? 我现在该做什么?

Shall we go for a walk? 我们去散步好吗?

Shall he come tomorrow? 你要他明天来吗?

should

情态动词Should表示"劝告"、 "建议 "时

这时should常译作 "应当"。如：

We Should learn about the computer and make full use of it.我们应该了解计算机并加以充分利用。

You should listen to the doctor’s advice.你应当听大夫的话。

You should study the article care fully. 你应当细，心学习这篇文章。

should还可以表示 "预测"、"可能"

They should be here by now.他们现在可能到了。

The reference book should be in the reading-room.这本参考书可能在阅览室里。

should有时表示说话人的感情如惊奇、愤怒、失望等

Why should I go? 我干吗要去?(不满)

I am sorry that he Should be so obstinate.我很遗憾，他竟这样固执。(失望)

It’s strange that it should be so hot today.很怪，今天怎么这么热。(惊奇)

should后跟动词的完成式时

这时句子指的是过去的事情。如果是肯定句，常说明某件事本应完成而未完成;如果是否定句，表示发生了不应当发生的事情.如：

You should have stopped at in red light. 你见了红灯本应该停车。

You Should not have gone back to work without the doctor’s permission.你不应当未经医生许可就回去工作。

He Should have come earlier. 他应早一点来。