[英语语法手册]情态动词－－have to
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。
[英语语法手册]情态动词--shall
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?