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.

他们用钥匙打开门进去了。

I looked everywhere for my keys and they were in the door all the time.

我到处找钥匙，没想到钥匙一直插在门上。

The key turned in the door, and I could hear the feet of the guards outside.

钥匙插在锁孔里，我可以听到外面卫兵的脚步声。

on the door例句

They had put a padlock on the door of his flat.

他们已经在他的公寓门上上了把挂锁。

There was an old rusting bolt on the door.

门上有一个生锈的旧门闩。

The lock on the door is rusty and won't open.

门上的锁锈住了。