at the door：在門邊
Someone is knocking at the door.
有人在敲門.
by the door：在門旁
By the door stood a man with a knife in one hand.
有個男人站在門旁,手里拿著小刀.
Who is the man that is standing by the door?
站在門邊的那個男人是誰?
其實(shí)他倆區(qū)別不大
如果非要區(qū)別的話,
① 通常放在固定詞組里德一定是at the door：
knock at the door敲門
lay at the door of 歸于
lay at their door 歸罪于……一系列的說法都是at the door而不是by the door
② 感覺上來講 at 有短暫接觸的意味（點(diǎn)）；by是大面積的依著、靠著（面）
如果不確定的情況下,都用at the door肯定沒錯