Learn Portuguese (Portugal): O jardim está atrás da casa.

The phrase for behind is atrás de. Because we want to say behind the house, we need the feminine word for the, which is a (since casa is feminine). In Portuguese, de and a combine into da.

Gardens and houses are permanent, so why use está instead of é?

While it is true that buildings and gardens do not move, Portuguese still uses the verb estar (to be) for physical locations. Whether a location is temporary or permanent, you will almost always use estar to say where something is.

Do all position words like atrás need a de after them?

Most basic location words in Portuguese are paired with de when they describe where something is in relation to another object. Just like atrás de (behind), you will see ao lado de (next to), em frente de (in front of), and debaixo de (under).

Why does atrás have an accent mark?

The acute accent on the á tells you to put the stress on that last syllable when speaking. Without it, the pronunciation rules of Portuguese would naturally stress the first syllable instead.