嚴(yán)格地說,have to不是情態(tài)動(dòng)詞,只不過它后面也是加動(dòng)詞原形,為了方便理解,把它當(dāng)作情態(tài)動(dòng)詞罷了.

have to是不是情態(tài)動(dòng)詞

have to可以視為情態(tài)動(dòng)詞詞組，但have to有時(shí)態(tài)和人稱的變化，比如第三人稱單數(shù)形式是has to，過去式是had to

You don't have to knock─just walk in.

不必敲門——進(jìn)來就是了。

He has to apply for a permit and we have to find him a job.

他得申請(qǐng)一個(gè)許可證，而我們得給他找份工作。

I had to keep in with the people who mattered.

我得和管事兒的人保持良好關(guān)系。

情態(tài)動(dòng)詞的介紹

1. 一般疑問句中情態(tài)動(dòng)詞要倒裝

比方說，She can go 的一般疑問句一定要是 Can she go.

She has to go 的一般疑問句 不是 “Has to she go”, 而是“Does she have to go”

have to 不倒裝，故而不符合英語句法對(duì)情態(tài)動(dòng)詞的要求

2. 情態(tài)動(dòng)詞沒有第三人稱形式

比方說，she can go, I can go, you can go, 都是 can

have to 就不同了，she has to go, 一定要用 has

3. 否定句中，否定詞在情態(tài)動(dòng)詞之后出現(xiàn)

比方說， she can't go (she can not go 的縮減形式）

have to 又不同了，必須說 she does not have to go, 這里要do-support, 而且否定詞在have to 前面。

當(dāng)然，還有其他區(qū)別，根據(jù)這幾條，我們基本可以判斷什么是情態(tài)動(dòng)詞，什么不是情態(tài)動(dòng)詞。