in his/her/their room
Si no fuera por él, Neil estaría en su cuarto, estudiando química.
If it weren't for him, Neil would be in his room, studying chemistry.
— La Sociedad de los Poetas Muertos (1989)
I heard her singing in her room.
Tomás está llorando en su cuarto.
Tomás is crying in his room.
He is playing in his room.
Tom comía todas sus comidas en su cuarto.
Tom ate all his meals in his room.
No encontré a mi padre ni en su cuarto ni en el jardín.
I couldn't find my father in his room or in the garden.