Just then, someone knocked at the front door.

就在那时，大门口有人敲门。

《牛津词典》

At last Mrs. Medlock knocked at a door, and when someone said, "Come in," they entered the room together.

最后，梅德洛克太太敲了敲门，当有人说“请进”时，他们一起进了屋。

youdao

Just last year, I was conducting a workshop when someone knocked at the classroom door.

就在去年，我正在教室里上研讨课的时候，突然有人敲门。

youdao

At that moment, someone knocked at the back door.

VOA: special.2010.06.12

Three o'clock in the morning, someone knocked on the door.

BBC: Lotfi Raissi transcript

As I was dozing off, someone knocked on the door.

NEWYORKER: The House Behind a Weeping Cherry

"Around 9pm that night the police knocked on the door and mentioned there had been an incident, and that someone was critically injured and taken to hospital, " he said.

BBC: Building on Byng Street where Mr Gorbuntsov was shot