"Estás en el jardín de él."
Translation:You are in his garden.
You could say that. It's just that "su" can represent so many people (his, her, its, their, your) that often people will say "... de [subject pronoun or name]" to make it clear to whom exactly something belongs.
"You are in the garden of him." was my answer. It got marked wrong unfortunately.
It's just not what you would say in english. Spanish doesn't use contractions like "Steve's [his] garden" it would instead say el jardín de Steve.
rspreng is right. The del contraction is only for the definite article el (the). It is never used with the subject pronoun él (he).
a garden is considered an area, not a specific point. So you are contained within the area, hence "in" not "at".
So in Spanish you use "jardín" either for a yard or a garden interchangeably?
Ennnuuur, the verb Estar (to be) conjugates: Yo Estoy, Tu Estas, El Esta, Nosotros Estamos, Ustedes (Ellos)Estan. all the "A's" in each word carry the accent mark, which I cannot make with my keyboard.