Learn Portuguese (Portugal): O senhor é português e ele não é alto. - You formal are Portuguese and he is not tall.

In Portugal, o senhor (literally "the sir" or "the gentleman") is a polite, formal way to say "you" to a man. Because it acts like a third-party noun, it uses the "he/she" verb form é instead of the informal "you" form és.

Could I just use "você" here instead of "o senhor"?

Grammatically, yes. você also means a formal "you" and uses the same verb é. However, in Portugal, o senhor (for a man) or a senhora (for a woman) is highly preferred because você can sometimes sound a bit too direct or blunt to locals.

Where exactly do we put "não" when making a sentence negative?

To say "not" in Portuguese, you simply put não directly in front of the verb you want to negate. In this sentence, it sits right before the second é to create não é (is not).

Why is "é" used to mean both "are" and "is" in the English translation?

The verb ser (to be) uses the exact same form, é, for both "he/she" and the formal "you". So, o senhor é translates to "you are", while ele é translates to "he is".

What would happen to the words "português" and "alto" if this sentence was about women?