she spends half an hour exercising in the gym every day,中exercise为什么加ing
问题描述：
she spends half an hour exercising in the gym every day,中exercise为什么加ing
答
spend sth. doing 是固定短语。
答
这里有个固定句式“
sb spend 一段时间,一段金钱 (in) doing sth
其中,in 可以省略,如果不省略,就更好理解,in 是介词,后面跟动名词