Learn Spanish (Latin America): Ellos están enojados porque pierden el partido. - They are angry because they lose the game.

Yes, when it means "because", porque is always one single word with no accent mark. If you want to ask "why?", you have to split it into two words and add an accent: por qué.

Why do we use están instead of son for "they are"?

We use están (from the verb estar) because being angry is a temporary emotion or state. If you used son (from the verb ser), it would mean they are angry people by nature.

Why does enojados end in -os?

Adjectives in Spanish have to match the people they describe. Since Ellos (they) is plural and masculine, the adjective enojado (angry) changes to its plural, masculine form: enojados.

I see the verb is pierden (they lose). Why isn't it perden?

The verb for "to lose" is perder. It is a stem-changing verb, which means the e in the middle changes to ie for most forms. So, for "they lose", it becomes pierden.

Does partido mean "party"?