Learn Spanish (Latin America): El niño gritó, y alguien corrió hacia el cuarto. - The boy screamed, and someone ran towards the room.

Just like "someone" or "somebody" in English, the Spanish pronoun alguien is always singular. You conjugate the verb in the third-person singular (the él/ella/usted form), which gives us corrió.

Since this lesson covers double negatives, how would I change this to say "and nobody ran"?

You would replace alguien with nadie (nobody). If nadie comes before the verb, it is simply y nadie corrió. However, if you move it after the verb, Spanish requires a double negative, making it y no corrió nadie.

Could I just use a instead of hacia here?

Yes, you could say corrió al cuarto (ran to the room). Using hacia specifically means "towards," emphasizing the direction of the movement rather than just the final destination. Be careful not to confuse it with hasta, which means "until" or "as far as."

Why do both verbs (gritó and corrió) have accent marks at the end?

They are both regular verbs in the preterite (simple past) tense for the third-person singular. The accent mark is crucial for showing who did the action and when! For example, gritó means "he screamed," but without the accent, grito means "I scream" in the present tense.