第一种表达：
knock on the door
He knocked on the door and waited for someone to answer.
他敲了敲门，等待有人回应。
第二种表达：
knock at the door 美式口语中常用来表示敲门声
She heard a knock at the door, so she opened it to see who was there.
她听见有人敲门，就把门打开看看是谁。
knock 英 [nɒk] 美 [nɑːk]
v.敲;击;(常为无意地)碰，撞;把…撞击成(某种状态)
n.敲击声;敲门(或窗等)声;捶击;敲击;撞击
第三人称单数： knocks 复数： knocks 现在分词： knocking 过去式： knocked 过去分词： knocked
扩展资料：
词语辨析
1、hit
to come against sth with force, especially causing damage or injury 指碰撞、撞击，尤指造成损伤：
例句：The boy was hit by a speeding car. 男孩被超速行驶的汽车撞倒了。
2、knock
to hit sth so that it moves or breaks; to put sb/sth into a particular state or position by hitting them/it 指打掉、敲动、打破、撞成…：
例句：Someone had knocked a hole in the wall. 有人在墙上打了个洞。
3、bang
to hit sth in a way that makes a loud noise 指大声地猛敲、砸：
例句：The baby was banging the table with his spoon. 小孩用调羹敲打着桌子。
4、strike
( formal ) to hit sb/sth hard 指猛烈地撞、碰、撞击、碰撞：
例句：The ship struck a rock. 船触礁了。
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