have to结构的划分

have to do结构中，have本来就不是情态动词，只不过have to do可以表示情态含义罢了，have本身是个实意动词，与不定式构成情态习语。正因为have to中的have不是情态动词，所以它有非限定形式（情态动词没有非限定形式）。

have to do的非限定形式：

He has had to do his homework.  had为过去分词。

Having to do his homework, he didn't go with us. having为现在分词。

I hate having to do homework. having为动名词。

He used to have to look after his younger brother. to have为不定式。

If he doesn't come tomorrow, you will have to do his job. have为不带to的不定式。