Question

【题目】进门前先敲门是礼貌的。
It's polite to the door before you come in.

Answer 1

【答案】knock；at
【解析】knock at the door敲门，to后跟动词原形，It是形式主语，后跟动词不定式做真正的主语，所以答案是knock； at 。