英语助动词中有基本助动词（be, have, do）和情态助动词（can, could, may, might, will, would, shall, should, must, ought to, dare, dared, need, used to）以及半助动词（因为它们有情态意义，而且后面接主要动词，同时它们有时态、人称、数的变化）。

半助动词根据是否可以用It...that-clause改写的依据可以分为subclass I 和 subclass II. 

SUBCLASS　Ｉ    不可以用 It...that-clause 改写 be about to,                be to, be able to,        to willing to, be apt to,         be bound to, be due to,         be going to,  be liable to,   be obliged to, be supposed to,  be sure to, had better,           had  best, have to,             have got to, come to,                     fail to, get to,                      tend to, etc.

SUBCLASS　ＩＩ     可以用 It...that-clause改写 be  certain to, be (un)likely to, appear to, chance to, happen to, seem to, turn out to, etc.