敲門
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" 指的是我輕輕的敲門，來告訴對方，我在門的另一邊。

knock (the) door
破門而入
"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樣破門而入。
S.W.A.T = Special Weapons And Tactics ['tæktɪks]
反恐特警

knocking someone up = to make someone pregnant
使某人懷孕
I really hope she isn't knocked up—I'm not ready to be a dad!
我希望她沒懷孕——我還沒有準備好做一個爸爸！

knock somebody out
狠揍某人
The boxer knocked out his opponent in the 3rd round.
這個拳擊手在第三輪擊敗了他的對手。

knock on wood
但愿有這種好運
I'll have a test tomorrow. Knock on wood.
我明天有考試。上帝保佑！

Don't knock it until you try it！
沒有嘗試之前，先別妄下定論。