have to通常指由于客觀原因則不得不做某事,意思為“不得不”.
e.g.It's dark now.I have to go home.
天黑了,我不得不回家.
It's raining,and I have to stay at home.
外面在下雨,我不得不呆在家里.
一、have to與一般情態(tài)動(dòng)詞的異同
相同點(diǎn)：
一般的情態(tài)動(dòng)詞不能單獨(dú)作謂語,后面必須接動(dòng)詞原形一起構(gòu)成謂語,have to 也是這樣.
不同點(diǎn)：
1、一般的情態(tài)動(dòng)詞沒有人稱和數(shù)的變化,而且所用的時(shí)態(tài)也受到一定的限制,而have to 有人稱和數(shù)的變化.可用于多種時(shí)態(tài)中：一般現(xiàn)在時(shí)中當(dāng)主語是第三人稱單數(shù)時(shí)要用has to,其余的人稱用have to,一般過去時(shí)中要用had to； 一般將來時(shí)中則要用will have to.
e.g.She has to go to school by bus.
她不得不乘公汽上學(xué).
You'll have to see the doctor if you get ill.
如果你生病了,就得去看醫(yī)生.
2、否定句和疑問句的構(gòu)成方式不同：
①一般的情態(tài)動(dòng)詞直接在后面加not構(gòu)成否定句,把這些情態(tài)動(dòng)詞提到句首就構(gòu)成一般疑問句.
②而have to 的否定句和疑問句的構(gòu)成往往要借助于助動(dòng)詞do的適當(dāng)形式或助動(dòng)詞will即have to,has to,had to和will have to的否定式分別為don't have to,doesn't have to,didn't have to和won't have to,疑問句是在句首加助動(dòng)詞do的相應(yīng)形式,句中還原成have to或把will放到句首,這里have to簡直就是一個(gè)十足的行為動(dòng)詞.
e.g.What does she have to do?
她必須做什么?
You didn't have to wait for me yesterday.
昨天你沒有必要等我
我記得沒有完成時(shí)