knock into sb和knock at

knock in to把东西敲（撞）进.,He knocked me in to the water.
knock at敲打某东西,He knocked at the melon to judge if it was fully ripe.

当knock作为名词的时候,介词用at,如a knock at the door
敲门声
动词是介词用on,如knock on the door 敲门

knock on 撞击撞出；
knock at 敲（门、窗等）
Who is that knocking at my door?
谁在敲我的门?

Knock at 与Knock on 的区别
前一个是正常状态下的敲,后一个非正常状态,如:用脚敲某物.
however 与 but 的区别 however在句子中是:无论怎样,它独立使用时是:然而.but是转折连词:但是,可是.
at the front,in the front,in front的区别
at the front:在前面(具体一个位置),
in the front:在物体的内部前面.
in front:在物体外部的前面.

efore entering（进门之前）是修饰knock at the door（敲门）的从句.enter本身是动词没错,但是它在从句里,并不是整个句子的主干.
分开来看就是：
主干：（祈使句）knock at the door
修饰1：（什么时候敲门?）before entering
修饰2：（礼貌语气）please

Knock on 指轻轻地敲 可以接身体的部位 强调的是结果.就如find是look for 的结果一样 而Knock at 表示一个过程 并且较猛.一般都接门.

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?
没什么大区别

shout at,对某人大喊,带有攻击性
shout to,向着某人喊,强调方向
knock at,敲,常用短语knock at the door,敲门
ask for,索取,如I asked my mum for money
祝楼主更上一层楼

1 I was playing the piano ,when there was a knock at the door.
2 I like climbing the mountains while my sister like swimming.
3 My mother was cooking while my father was watching TV.
4 When she got home,she did her homework.
5 When I walk in the street ,I saw an accident.

首先,是differences,拼错了...
开始正题：
knock at是敲门声
knock on是正在敲门,是一种动作

It was evening.Jane was watching TV at home.Suddenly she heard someone knocking at the door.Then Jane went to open the door.A man came in and knock her off.When Jane was sttruggling shen knock a big vase off.Her neighbour heard the noise and came to help her.Finally Jane was survived.

knock:[ nɔk ]
n.敲,敲打,敲门
v.敲击,互撞,攻击
Examples:
1.He knocked a big hole in the wall.
他把墙撞出了一个大洞.
2.Did you hear the knocks at the door?
你听到敲门声了吗?
3.Someone is knocking at the door.
有人在敲门.
4.I often feel that politicians should have their heads knocked together,like naughty children.
我时常觉得应该像教训淘气孩子那样让政治家停止争吵.
5.She knocked the bottom out of our argument.
她把我们的论点驳得体无完肤.
6.Knock it off,kids,I'm trying to sleep!
孩子们,别闹了,我要睡觉了!

knock on 敲(门、窗等);撞
knock at 敲(门,窗等)
Knock on the door before you enter.
进屋之前要先敲门.
When I fell I got a terrible knock on the head.
我摔倒时头撞得很厉害.
He got a nasty knock on the head.
他头部遭到重重一击.
Did I hear a knock at the door?
是有人敲门吗?
I was wakened by a knock at the door.
敲门声把我吵醒了

knock on 敲(门、窗等);撞
knock at 敲(门,窗等)
Knock on the door before you enter.
进屋之前要先敲门.
When I fell I got a terrible knock on the head.
我摔倒时头撞得很厉害.
He got a nasty knock on the head.
他头部遭到重重一击.
Did I hear a knock at the door?
是有人敲门吗?
I was wakened by a knock at the door.
敲门声把我吵醒了

knock [+at/on] 意思相同。“敲门（或窗等）”，没有区别。
例句：
He heard someone knocking at the door .
他听见有人在敲门。
Please do not enter before knocking on the door.
进来前请先敲门。

knock at敲打某东西,He knocked at the melon to judge if it was fully ripe.
knock in to把东西敲（撞）进.,He knocked me in to the water.
后面有door时敲门用knock at,knock on都可以
后面没有door时用knock
He came in without knocking.他没敲门就进来了
I knocked on (at)the door,but there was no answer.我敲了门,但没有回应