Be have to do 有两种划分办法
①have to作半情态动词，do是实意动词因此have to do整个作复合谓语
②have是谓语，to do作宾语，可是如此分开have本来没不能不的意思，如此难道不会翻译不完整吗？
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We shall have to manage with these old pencils
这个例句shall是情态动词，have to又是半情态动词，这如何会出现两个呢？除非have to不是情态动词，这不就推翻了①的结论吗？
have to do结构中，have本来就不是情态动词，只是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。正由于have to中的have不是情态动词，所以它有非限定形式（情态动词没非限定形式）。
have to do的非限定形式：
He has had to dohis homework. had为过去分词。
Having to do hishomework, he didn't go with us. having为目前分词。
I hate having todo homework. having为动名词。
He used to haveto look after his younger brother. to have为不定式。
If he doesn'tcome tomorrow, you will have to do his job. have为不带to的不定式。
