A1 · The word agrees with the speaker or the thing

Agreement — gender, you, thank you

Obrigado is about you, not them. Articles and possessives match the noun: o meu pai, a minha mãe. And 'you' is a relationship: tu with friends, o senhor / a senhora with older strangers.

• Obrigado. / Obrigada.

  Thank you (man speaking / woman speaking).

• O meu café. A minha bica.

  My coffee. My espresso. (noun gender)

• Como está, senhora Silva?

  How are you, Mrs Silva? (polite)