Knock on:
敲（门knock at、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？前者表示一种正常情况下的敲门,后者则表示非正常情况下的敲门如小孩子喜欢用背去敲门.而常人则抬手齐耳处敲门.
knock on有一种持续性，常有引申意义（如击打某人的内心），knock at强调的是对象，敲门、窗之类的用后者比较好