door与介词和动词的搭配问题
1. 若表示“在门口”，通常与介词at搭配。如：
Someone’s knocking at the door. Can you answer it? 有人敲门，你去开一下好吗?
若表示“敲门”，其前既可用搭配介词at也可用介词on。如：
Somebody was beating at [on] the door. 有人不停地敲门。
Someone knocked at [on] the door at midnight. 午夜时有人敲门。
They heard a tap at [on] the door. 他们听见轻轻的敲门声。
若用介词in，则需要根据具体语境来确定其含义。如：
The gun shot a hole in the door. 这枪在门上射出一个洞。
She got her fingers caught in the door. 她的手指让门给夹了。
Directly I walked in the door I smelt smoke. 我一进门，就闻见了烟味。
He stood in the door for several minutes before deciding whether he’d stay. 他站在门里边好几钟才决定是否留下来。
2. by the door与through the door均可指“通过门”，但前者侧重方式，后者侧重实实在在地“通过”。如：
We couldn’t get the piano through the door. 我们无法将钢琴搬过这道门。
The thief slipped out by the back door. 那个贼偷偷从后门溜出去了。
汉语说“走后门”，指的是通过不正当的手段来达到某种个人目的，有趣的是，在英语中也有类似用法，用by the back door或through the back door均可。如：
He used his influential friends to help him get into the civil service by the back door. 他利用那些有权有势的朋友帮助他走后门，到政府行政部门任职。
3. open the door与answer the door均可表示“开门”，但有区别：前者close the door(关门)相对，指把门打开;后者指听到门铃响或敲门声而去开门。如：
He opened the door and went out. 他打开门就出去了。
Will you answer the door? There’s someone knocking. 请你去开门，有人在敲门。
4. show sb the door与show sb to the door只差一个介词to，但意思区别很大：前者意为指着门要某人出去，即把某人撵走或下逐客令;后者意为送某人到门口，即相当于汉语中的送客。如：
After having insulted his host, he is shown the door. 他侮辱了主人，结果被轰了出去。
Whenever a guest leaves, my father will show him to the door. 每当客人离开时，我父亲都把客人送到门口。
show sb to the door 也可说成 see sb to the door。如：(www.yygrammar.com)
Goodbye, Mr Carter. My secretary will see you to the door. 再见，卡特先生，我的秘书将送你到门口。
5. 习语 (from) door to door的本义是“从一家门口到另一家门口”，也就是“逐门逐户”“挨家挨户”，相当于 (from) house to house。如：
The journey takes about an hour, door to door. 从这家到那家的路程要用一小时。
We went from door to door collecting money for charity. 我们挨家挨户地募集慈善捐款。