How come this one doesn't need an object pronoun?
Because it is stated in the sentence - it's usually the indirect object pronoun that gets included "a mí me gusta"
My answer, "Él responde a mi letra." marked as wrong, would someone comment?
Letra usually refers to lyrics or handwriting. Carta is a better match for a paper letter that you send to someone.
A letter of correspondence is "carta". A letter of the alphabet is "letra".
Same thing happened to me?
I omitted a and was accepted.
I dont understand the a part. What is the significance of 'a'
"A" is like "to"
it was clearly a typo