动词ing开头是 动名词作主语的句型。动名词相当于名词，可以作主语，而动词原形不能直接用作主语。例如：

1. Studying hard is very important.

2. Helping others is good.

3. Doing lots of exercise is good for health.

4. Walking along the street, he caught sight of an old friend of his.