Sometimes prepositional phrases can come between the subject and the verb. If the object of the preposition is singular and the subject is plural, or if the object of the preposition is plural and the subject is singular, there can be confusion in making the subject and verb agree.

In the first example you might think that “doors” is the subject because it comes directly in front of the verb “are”. However, “doors” is not the subject because it is the object of the preposition “to”. The subject of the sentence is “key”, so the verb should be “is”. In the second example you might think that “door” is the subject because it comes directly in front of the verb “is”. You should recognize in this example that “door” is not the subject because it is the object of the preposition “to”. Because the subject of the sentence is “keys”, the verb should be “are”.