当动词以动名词形式出现在句首作主语时(这时它的用法同名词的用法),就是动词ing,如：Shopping is the favorite activity of most women.
另外一种情况是用动词的现在分词形式Ving作伴随状语时,如：Knowing her for many years (=After he had known her for many years),he decided to propose to her.这种情况一般能把它还原成从句的形式
什么时候动词为首单词时是ing形式 什么时候又不是ing形式能 打个比方说明一下