veinna2002
have to是一个比较特殊的情态习语，它也有情态动词的功能，但却与其他的情态动词不同。因为它有人称、数和时态的变化，其否定式和疑问式的构成方式也与情态动词不一样。
严格地说，have to不是情态动词，只不过它后面也是加动词原形，为了方便理解，把它当作情态动词罢了。像need和dare to，既可以当情态动词，又可以当实义动词，当作为情态动词时，词形不变，而作为实义动词时，则会相应地改变词形。
Have to的用法
一、have to的陈述句形式
A、肯定式：have to +动词原形
I have to tidy my room.我得整理房间。
B、否定式：don't (doesn't) + haveto +动词原形
You don't have to go if you don't want to.如果你不想去，你就不必去。
二、have to的一般疑问句形式及简略答语
have to的一般疑问形式必须借助助动词do或does，例如：
Do you have to look after your sister?你得照看你妹妹么？
Yes, I do./ No, I don't.是的，我得照看。／不，我不必照看。
三、have to的特殊疑问句形式
1、What do you have to do on Sundays?在星期天你得干什么？
2、Where do they have to work?他们必须在哪里工作？
WaimanTong
have to的用法如下：have to 后接动词原形。
Have to是情态动词 ,意思是“必须、 不得不”。have to的三单形式是 has to 。
I have to stay at home .
我不得不呆在家。
My mother has to get up early.
我妈妈不得不起床早。
have to /has to 的否定句分别在have to 或has to 前加don't 或doesn't.
一般疑问句在句首加do或does。
例如：
1. They have to go to Shanghai. 他们不得不去上海。
否：They don't have to go to Shanghai.
一般疑问句：Do they have to go to Shanghai？
Yes, they do.
No, they don't.
2. Tom has to study hard. Tom 不得不努力学习。
否：Tom doesn't have to study hard.
一般疑问句：Does Tom have to study hard？
Yes, he does.
No, he doesn't.
小胶带儿
had to是have to的过去形式，意思上没多大差别，只是had to指的是过去， have to指的是现在一般情况。例如： I have to do my homework now I had to do my homework last night, so I couldn't go shopping with you
