Learn Portuguese (Portugal): Elas estão tristes no quarto.

We use the verb estar (here conjugated as estão) because it describes temporary states and locations. Being sad is a temporary feeling, and being in the bedroom is a temporary location. If we used são (from the verb ser), it would mean they are permanently sad people by nature.

Why is there an 's' on the end of tristes?

In Portuguese, adjectives must match the number of things or people they describe. Because Elas is plural, the adjective triste (sad) gets an 's' added to become tristes. Note that triste is gender-neutral, so it doesn't change to an 'a' or 'o' for masculine or feminine.

How does the word no translate to "in the"?

The word no is a mandatory contraction. It combines the preposition em (in, on, or at) with the masculine singular word for 'the', which is o (because quarto is a masculine noun). Whenever em and o meet, they merge to form no.

Can Elas be used if the group in the bedroom includes both men and women?