Question

knock at the door为什么不是knock on the door

Answer 1

at强调一个点,on强调面 knock at the door 和knock on the doo r 都是敲门的意思，二者区别不大，

at 常和knock 名词形式连用
on常和knock 动词形式连用如果knock用作名词时，用at.
没有太绝对的区别，只是一般这样使用
eg:
Did I hear a knock at the door?

Please do not enter before knocking on the door. Knock on: 敲（门、窗等）
例句与用法:
1. He accidentally knocked on (the pass from Jones).
他非故意犯规, 用手向前击了（从琼斯传来的）球.
2. Jim sat up when he heard the knock on his bedroom door.
吉姆听到敲寝室门的声音时便坐了起来。
3. Please do not enter before knocking on the door.
进来前请先敲门。

Knock at: vt. 敲

例句与用法:
1. There was a furious knock at the door.
有人在猛烈地敲门。
2. Did I hear a knock at the door?
是有人敲门吗?
3. Did you hear the knocks at the door?
你听到敲门声了吗？