敲门
knock on the door
knock at the door
"Knocking on" implies that I made a slight tap on the door to alert anyone who is on the other side of the door.
"Knocking on" 指的是我轻轻的敲门，来告诉对方，我在门的另一边。

knock (the) door
破门而入
"knocking (the) door" means that I broke upon the door like a S.W.A.T member.
"knocking (the) door" 指的是像反恐特警一样破门而入。
S.W.A.T = Special Weapons And Tactics ['tæktɪks]
反恐特警

knocking someone up = to make someone pregnant
使某人怀孕
I really hope she isn't knocked up—I'm not ready to be a dad!
我希望她没怀孕——我还没有准备好做一个爸爸！

knock somebody out
狠揍某人
The boxer knocked out his opponent in the 3rd round.
这个拳击手在第三轮击败了他的对手。

knock on wood
但愿有这种好运
I'll have a test tomorrow. Knock on wood.
我明天有考试。上帝保佑！

Don't knock it until you try it！
没有尝试之前，先别妄下定论。