b.以have为中心成分，如：
　　　　had better/best　 have got to
　　　　have to

　　　c.以seem等为中心成分，如：
　　　　seem to 　　turn out to be
　　　　appear to 　fail to
　　　　happen to 　tend to

　　2.半助动词既可是限定形式（体现时态，语态特征），也可是非限定形式（不定式或现在分词）。半助动词后的主动词都用带to 的不定式，这种不定式可以是一般形式、进行式或完成式，如：
　　　He appears to have many friends.
　　　He seems to be enjoying himself at the party.
　　　Having to wait for the bus, I spent the time trying to telephone Nancy.
　　　You are supposed to have finished your term paper by Sunday evening.