have to在句子一般什么作用?谓语?

问题描述:

have to在句子一般什么作用?谓语?
比如一个句子:I have to do it。
谓语是do 还是 have to?have to 是作修饰的么?

have to 相当于情态动词 must
have to+动词原形 共同做谓语