1. 两种形式：动名词做主语一是置于句首；二是置于句尾，将先行词it置于句首（即用It
　+ be + … + V-ing句型），如：
　　Burning takes place much faster in pure oxygen than in air.
　　It is of great importance fighting against pollution.

2. 特别用在下面的句子结构中：
　a. It is 后接no use, no good, fun等名词，如：
　　It is no use learning theory without practice.

　b. It is 后接useless, nice, good, interesting, worthwhile等形容词，如：
　　It's worthwhile taking this into consideration.

　c. 在there + be + no + V-ing句型中，如：
　　There is no joking about such matters.

3. 用在复合结构中：动名词如有自己的逻辑主语则一起构成动名词的复合结构，它的逻辑
　主语多用名词的所有格或物主代词，如：
　　His coming here helped us a lot.
　　Tom's escaping from the prison made trouble for the jailer.