钥匙在门上用in。in虽然也翻译为在门上，但钥匙是插在门里面，所以用in。例如，The key is in the door。
in the door例句
They put the key in the door and entered.
I looked everywhere for my keys and they were in the door all the time.
The key turned in the door, and I could hear the feet of the guards outside.
on the door例句
They had put a padlock on the door of his flat.
There was an old rusting bolt on the door.
The lock on the door is rusty and won't open.