Learn Spanish (Latin America): El cojín rojo está detrás del mueble. - The red cushion is behind the piece of furniture.

The phrase for "behind" is detrás de. Whenever the preposition de is followed immediately by the masculine singular article el ("the"), they must combine to form the contraction del.

Can I just say detrás el mueble without the de?

No, you need the de. Complex prepositions of place like detrás (behind), enfrente (in front), and encima (on top) require de when you specify the object they are relating to. It translates literally to "in back of".

Why do we use está instead of es here?

We use the verb estar (conjugated here as está) to talk about physical location. Even though a piece of furniture doesn't move often, Spanish always uses estar rather than ser to say where something is located.

In English, "furniture" is uncountable. Why is mueble singular in this sentence?

In Spanish, un mueble is a countable noun that refers to one single piece of furniture, like a specific cabinet, desk, or sofa. If you wanted to talk about a group of furniture, you would use the plural form, los muebles.