Knock on:
敲（門、窗等）
例句與用法:
1.He accidentally knocked on (the pass from Jones).
他非故意犯規(guī),用手向前擊了（從瓊斯傳來的）球.
2.Jim sat up when he heard the knock on his bedroom door.
吉姆聽到敲寢室門的聲音時便坐了起來.
3.Please do not enter before knocking on the door.
進來前請先敲門.
Knock at:
vt.敲
例句與用法:
1.There was a furious knock at the door.
有人在猛烈地敲門.
2.Did I hear a knock at the door?
是有人敲門嗎?
3.Did you hear the knocks at the door?
你聽到敲門聲了嗎?
沒什么大區(qū)別