have to后跟动词原形，表示客观需要做的事情，意思是“必须”“不得不”，有人称、时态和数的变化，与情态动词must近义。例句：He had to go to the hospital for treatment.他不得不去医院接受治疗。
一、have to的陈述句形式
A、肯定式：have to+动词原形
1、I have to tidy my room.
我得整理房间。
2、She has to help with the washing.
她得帮忙洗衣服。
B、否定式：don't(doesn't) + have to + 动词原形
1、You don't have to go if you don't want to.
如果你不想去，你就不必去。
2、He doesn't have to stay at home all day.
他不必整天呆在家里。
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：
1、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。
2、Does Jim have to do his homework?
吉姆必须做家庭作业么?
Yes, he does. / No, he doesn't.
是，他必须做。/不，他不必做。
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、Why does she have to move to Paris?
她为什么得迁往巴黎？
3、Where do they have to work?
他们必须在哪里工作？
四、have to可用于各种时态
A、一般现在时：
I have to visit Mr Wang tomorrow.
明天我得去拜访王先生。
B、一般过去时：
That night we had to walk home because there was no bus.
那天晚上我们不得不步行回家,因为没有公共汽车。
C、一般将来时：
We'll have to ask Zhang Ming instead.
我们得请张明代替。
D、与may连用：
I think he may have to help his Dad in the garden.
我想他可能得在花园里帮他爸爸干活。