his/her letter
Lorsqu'il vit sa lettre, il se sentit un peu mal à l'aise.
When he saw her letter, he felt a little uncomfortable.
Sofi, aide Pepe à écrire sa lettre sur la table.
Sofi, help Pepe write his letter on the table.
— Roma (2018)
J'ai simplement modifié sa lettre, pour donner l'illusion d'un testament.
I simply altered his letter to give the impression that it was a will.
— Decision to Leave (2022)
I must reply to his letter.
Did you receive his letter?
You must reply to his letter.