Learn Portuguese (Portugal): Eu não sou alto.

In Portuguese, you make a sentence negative by putting the word não (not) directly before the verb. Since the verb here is sou (am), não goes right in front of it.

Would this sentence look different if a woman were speaking?

Yes! Adjectives in Portuguese change to match the gender of the person they describe. Alto (tall) is the masculine form. A woman would change the ending to an -a and say: Eu não sou alta.

Why do we use sou for "am" instead of estou?

Portuguese has two verbs for "to be". We use ser (conjugated here as sou for eu) to describe permanent or unchanging characteristics, like someone's height. You use the other verb, estar (estou), for temporary states like being hungry or tired.

Do I have to include the word eu (I) in this sentence?

No, it is optional. Because the verb form sou is only ever used for eu, the "I" is already understood. You will often hear native speakers drop the pronoun entirely and simply say: Não sou alto.