1.直接位于句首做主语.例如：
Swimming is a good sport in summer.
2.用 it 作形式主语,把动名词(真实主语)置于句尾作后置主语.
动名词做主语时,不太常用 it 作先行主语,多见于某些形容词及名词之后.例如：
It is no use telling him not to worry.
常见的能用于这种结构的形容词还有：better,wonderful,enjoyable,interesting,foolish,difficult,useless,senseless,worthwhile,等.
注意：important,essential,necessary 等形容词不能用于上述结构.
3.用于“There be”结构中.例如：
There is no saying when he'll come.很难说他何时回来.
4.用于布告形式的省略结构中.例如：
No smoking ( =No smoking is allowed (here) ).
No parking.
、
在“It is no use...”,“It is no good...”,“It is fun...”,“It is a waste of time...”等句型中,通常用动名词作真实主语：
某些动词后出现非限定性动词时只能用动名词作宾语,不能用不定式.常见的此类动词有：advise,allow,permit,avoid,consider,enjoy,finish,give up,cannot help,imagine,include,keep,keep on,mind,miss,put off,delay,practise,resist,suggest,depend on,think about,set about,succeed in,worry about,burst out,insist on,can’t stand,be used to,get used to,devote…to…,look forward to,pay attention to,get down to等.
