his/her diary
Ella cerró lentamente su diario.
She slowly closed her diary.
En su diario, lee, lee tú mismo su diario.
In his diary, read, read his diary yourself.
— Divorcio a la italiana (1961)
Él decidió escribir en su diario todos los días.
He decided to write in his diary every day.
Decidió escribir todos los días en su diario.
She decided to write in her diary every day.