watch sb do sth 还是to do还是doingThe teacher often watches us playing解释一下有区别么?是不是看他正在玩必须用ing,其他时态没区别?

没有watch sb to do.
只有watch sb doing 和watch sb do；前者正如你所说的，意为“观看某人正在做某事”；后者则是
“观看某人做了某事”

want to do sth想要做某事 I wantI want an apple. like doing sthteacher tells us about the exam. often watches Wangwang playing the

watch sb do sth 表示看到了某人做了某事或经常做某事；
watch sb doing sth 表示看到某人正在做某事