同学你好，该知识点来自沪江网校《英语零基础直达PETS三级/公共英语三级【年中特惠班】》的课程，想要更系统的学习，欢迎进入课程学习。不仅可以和更多的同学一起学习，而且还有老师、助教随时的学习指导和知识点解答哦。

同学你好~
at强调一个点,on强调面 knock at the door 和knock on the door 都是敲门的意思，二者区别不大。
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？

~祝同学学习愉快~~么么哒~~
同学有疑问可以继续提问喔；
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