1.It's no use (counting) your chickens before they are hatched.
It's no use +doing 固定搭配
2.（ Is it really necessary to spend an hour in the bathroom every morning ）combing your hair?
combing your hair?修饰前面整个句子,用doing形式,是一种语法,忘记名称了
3.He wondered if he could slip out of the lecture hall without anyone (noticing).
without/with+doing
4.(Sitting) there like that,you remind me ofyour father.
sit没有被动
5.I wrote a letter to the car dealer,(explains) what had happened to my new car.
有and才能用explain,这里explaining也能用