半助动词(semi-auxiliary)是兼有助动词和主动词双重词性的动词形式，它有三种类型：be- 型、have- 型和seem- 型， 它们的词项列表如下： 

    be- 型：be able to / be about to / be apt to / be bound to / be certain to / be going to / be liable to / be obliged to / be supposed to / be sure to / be (un)likely to / be to / be willing to , etc

    have- 型：have to / have got to / had better , etc

    seem- 型：seem to / appear to / come to / fail to / get to / happen to / tend to / turn out to ,etc

    （摘自章振邦主编：《新编高级英语语法》）