do怎么变成doing?

什么时候when sb do sth 可变成when doing sth 感官动词变成被动语态后要加-to do或-doing do some doing的结构怎么用? saw sb doing/do怎么用 to do 和doing 怎么分别? Tom is doing his homework.变成否定句怎么变? see sb.doing sth.变成被动语态怎么变呢? found sb doing/do sth 到底怎么区分... 接下来怎么做阿,find to do还是find doing 一个动词放句首,怎么判断用doing还是to do? wait to do 和wait doing怎么用? 怎么能区分to do ,doing、、不定式,动名词的用法?