Order changed why have you locked the door? I don't have key - no. Needs an article why have you locked the door? I don't have a key - correct why have you locked the door? I don't have the key - correct They mean different things. a key = any key, as if this lock has many keys and you don't have any of them with you. the key = THE one key you are talkiing about. Usually interpreted to mean your key.