1.半助动词在功能上介于主动词和助动词之间，本身有词汇意义，可分为三类：
a.以be为中心成分，如：
be about to be going to
be bound to be able to
be due to be sure to
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.
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Nouns
