his/her/your letter
Ella leyó su carta una y otra vez.
She read his letter over and over again.
Tengo que responder a su carta.
I have to reply to your letter.
Jackson no recibió una respuesta a su carta.
Jackson did not receive a response to his letter.
Espero con ganas a recibir su carta.
I look forward to receiving your letter.
Creo lo que está escrito en su carta.
I believe what is written in your letter.
Recibí su carta hace unos días.
I received your letter a few days ago.