have to可以视为情态动词词组,但have to有时态和人称的变化,比如第三人称单数形式是has to,过去式是had to
You don't have to knock─just walk in.
不必敲门——进来就是了。
He has to apply for a permit and we have to find him a job.
他得申请一个许可证,而我们得给他找份工作。
I had to keep in with the people who mattered.
我得和管事儿的人保持良好关系。
1. 一般疑问句中情态动词要倒装
比方说,She can go 的一般疑问句一定要是 Can she go.
She has to go 的一般疑问句 不是 “Has to she go”, 而是“Does she have to go”
have to 不倒装,故而不符合英语句法对情态动词的要求
2. 情态动词没有第三人称形式
比方说,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,否定词在情态动词之后出现
比方说, she can't go (she can not go 的缩减形式)
have to 又不同了,必须说 she does not have to go, 这里要do-support, 而且否定词在have to 前面。
当然,还有其他区别,根据这几条,我们基本可以判断什么是情态动词,什么不是情态动词。