主语外置(Extraposition)是指借助形式主语it把主语移到句末的现象。主语从句和不定式是最典型的外置主语:
It worries me that he hasn't phoned.
It is unclear why she told him.
It would be pointless to resist.
Would it be better to pay now?
It is incredible what a fuss some people make.
动名词和名词词组偶尔也可以外置,但是比较少:
It would be no use complaining.
It's extraordinary the amount of beer he puts away.