In most of Brazil, você is what you say to almost everyone — friends, colleagues, often the boss (in some regions, friends say tu). O senhor (to a man) and a senhora (to a woman) add respect and distance. You'll hear them with older people, senior people and clients, and whenever someone is addressed formally for the first time. The plural is os senhores / as senhoras.

The grammar is the easy part — the verb is the same as for você:

• O senhor pode assinar aqui? — Could you sign here?
• A senhora gostaria de um café? — Would you like a coffee?

For your, o seu / a sua works, but do senhor / da senhora leaves no doubt: o escritório da senhora. As an object, simply repeat it: Posso ajudar o senhor? Vou enviar para a senhora. Te belongs with você — pairing it with o senhor mixes two registers.

In writing, the titles shorten to Sr., Sra. and Dr. / Dra. Doutor / doutora is used for physicians and lawyers, and in many workplaces as a courtesy title for senior people.

The way out of formality is one sentence, usually offered by the person being called o senhor or a senhora: Pode me chamar de você. — You can call me você.