Final answer:
To change the sentence 'Alguien está en la puerta' into a negative sentence, use 'No hay nadie en la puerta'.
Step-by-step explanation:
The sentence Alguien está en la puerta translates to Someone is at the door. To change this sentence into a negative sentence, we need to use the negative form of alguien, which is nadie. So, the correct negative sentence would be No hay nadie en la puerta, which translates to There is no one at the door.
Learn more about Negative sentences in Spanish