Learn Portuguese (Portugal): Há um professor e três alunos na sala. - There is one teacher and three students in the living room.

It can be both! In Portuguese, the number one and the masculine word for "a" or "an" are the exact same word: um. Because we are counting teachers and students here, it translates best as "one".

We learned that some numbers change based on gender. Does três change if the noun is feminine?

No, the number três (three) stays exactly the same for both masculine and feminine words. Only the numbers one (um or uma) and two (dois or duas) change to match the gender of the noun.

Why do we use Há for "There is" instead of a word like "Lá"?

The verb haver is used to say that something exists, just like "there is" or "there are" in English. Há is the specific form we use for both singular ("there is one teacher") and plural ("there are three students").

Why does the sentence use na instead of just em for "in"?

In Portuguese, you have to combine the word for "in" (em) with the word for "the". Because sala is a feminine word, we use a for "the". When you squeeze em and a together, they become na ("in the").