I'll tell him/her/it
Se lo diré cuando esté todo listo.
I'll tell you when everything is ready.
— El prisionero de las montañas (1996)
I'll tell you everything.
Tan pronto como llegue se lo diré.
As soon as I arrive, I'll tell him/her.
Basta, pórtate bien o se lo diré a tu padre.
That's enough, behave yourself or I'll tell your father.
— Cuentos de Tokio (1953)
Se lo diré a Tom.
I'll tell Tom.
I won't tell him/her.
All right, I'll tell him.
I'll tell everyone.