Learn Portuguese (Portugal): As mesas não são altas.

As is the feminine plural definite article. Because the noun mesas (tables) is feminine and plural, the word for "the" must match it perfectly. If we were talking about a masculine plural noun, we would use Os.

Why is the negative word não placed before the verb?

In Portuguese, you make a sentence negative by putting não directly in front of the verb. While English says "are not", Portuguese structures it as "not are" (não são).

Why do we use the verb form são in this sentence?

São means "are" when talking about "they" (plural). Since we are talking about multiple tables, we use the "they" form of the verb ser (to be) instead of the singular form é (is).

Why does the adjective altas end with an "s"?

Adjectives (describing words) in Portuguese must match the gender and number of the noun they describe. Since mesas is feminine and plural, the base adjective alto (tall) changes to its feminine plural form, altas.