"Tua" and "sua" are possessive pronouns related to personal pronouns "tu" and "ele/ela", respectively. "Sua" is also used in relation to the pronoun "você". In spoken portuguese "tu" and "você" mean the same thing: YOU. However, grammatically, "tu" is a personal pronoun and "você" is a title (pronome de tratamento), which is used as a personal pronoun. Problem is, when you use "você" you have to use the same possessive pronouns that "ele/ela" require (as if "você" was the 3rd person). Then, the both sentences are correct (Someone reads someone else's letter). The difference is: in the first sentence the letter's owner is treated by "tu" and in the second one "você". Tu tens uma carta. Ela lê a tua carta. Você tem uma carta. Ela lê a sua carta.

Hii Portuguese from brazil (dialect) they don't use "tua" they allways use "sua" or "seu"... In Portuguese from portugal people usually use "Sua" for formal settings or for people more old...but all mean "you" Exemple: Avó eu gosto da sua roupa =Grandmumy i like ur clothing... For a friend : Eu gosto da tua roupa = I like ur clothing... Brazilian people don't use Tua...in portuguese from Portugal we do difference of formal and not formal and when it's for old people