Which episode features Natsu joining Fairy Tail and Erza teaching him to write? I know there was one episode where it says how Natsu joined Fairy Tail and then he found some mission and Erza started teaching him reading and writing, but I forgot which one it was....
I think you're referring to Episode 27. Also note that that was an anime only scene, and you aren't quite remembering it right. Natsu already knew how to read thanks to Igneel. Erza just expanded his vocabulary.