Subject + 敲门
敲门 (qiāomén) means "to knock on the door," a common verb-object compound in Chinese. 敲 means "to knock" and 门 means "door." It can also be used more broadly, as 敲 can combine with other objects to mean knocking or striking something.
有人在敲门,你去看看是谁。
Yǒu rén zài qiāo mén, nǐ qù kànkan shì shéi.
Someone is knocking at the door — go see who it is.
我敲了半天门,里面都没有人回应。
Wǒ qiāo le bàntiān mén, lǐmiàn dōu méiyǒu rén huíyìng.
I knocked on the door for a long time, but nobody inside responded.
进办公室之前请先敲门。
Jìn bàngōngshì zhīqián qǐng xiān qiāo mén.
Please knock before entering the office.