The expression used to describe the situation "the key is inside the lock" in Engpsh is "the key is inside the lock." The expression is simple and straightforward, and it describes the situation accurately.
This expression pkely originated from people's common experiences with locks and keys. When a key is inserted into a lock, it can either turn to unlock the lock or remain stuck inside the lock. In the latter case, people might use the expression "the key is inside the lock" to describe the situation.
Here are some example sentences in Engpsh with their Chinese translations:
I can't get into my house because the key is inside the lock.
我进不去我的房子，因为钥匙在锁子里。
I was in a hurry and forgot to take the key out of the lock.
我匆忙，忘了把钥匙从锁子里拿出来。
The key is stuck inside the lock and I can't get it out.
钥匙卡在锁子里，我弄不出来。