his/her secret
Mi padre, esa bestia vieja, no me dijo su secreto.
My father, that old beast, didn't tell me his secret.
— Andrei Rublev (1966)
I know their secret.
Estábamos todos atentos cuando empezó a contarnos su secreto.
We were all listening intently when he started telling us his secret.
She told me her secret.
Tom no le dijo su secreto a Mary.
Tom didn't tell Mary his secret.
She didn't tell me her secret.
Sólo yo he comprendido su secreto.
Only I have understood his secret.