Learn Portuguese (Portugal): O cão está fora da casa.

The phrase for "outside" is fora de. Because you are saying "outside the house" (a casa), you must combine the word de (of) with a (the). In Portuguese, de + a always merges to become da.

Do we always need de after location words like fora?

Yes! Just like the other location phrases you are learning—such as ao lado de (next to) or em cima de (on top of)—fora needs de to connect it to an object. You cannot just say fora a casa.

Why use está for "is" instead of é?

The verb estar (where está comes from) is used to talk about locations. Whenever you are stating where a person, animal, or object is physically located, you must use estar. You would use é (from the verb ser) for permanent traits, like saying the dog is a certain breed.