Learn Portuguese (Portugal): Você não é a professora.

In Portuguese, to make a sentence negative, you simply put não directly before the verb. So while in English we say "are not", in Portuguese it is always structured as "not are" (não é).

If você means "you", why do we use the verb é, which usually means "is"?

Even though você translates to "you", grammatically it acts like a third-person pronoun (just like ele or ela). Because of this rule, it must be paired with é, which is the third-person singular form of the verb to be.

When should I use você instead of tu to say "you"?

Você is used in formal situations or with people you do not know well, such as an adult stranger or someone in a professional setting. Tu is informal and used with friends, family, and children. Note that in Portugal, people often use titles like o senhor or a senhora instead of você to be extra polite.

Would this sentence change if I were talking to a male teacher?

Yes! Because Portuguese nouns have genders, you would change the feminine a professora (the female teacher) to the masculine o professor (the male teacher). The rest of the sentence (Você não é) stays exactly the same.