Definite and indefinite articles

Portuguese usually drops the indefinite article for a bare profession, nationality, marital status or religion after ser: sou estudante, ela é médica. Sou um estudante is not wrong, just less natural unless you are stressing it. Add um or uma back the moment an adjective describes the noun.

De, em and a do not sit beside the definite article in an ordinary sentence: they fuse into one word. De + o is do, em + a is na, a + o is ao, and the plurals follow (dos, nas, aos). Writing em o Brasil is a mistake every time.

A Portuguese article agrees with its noun, never with the speaker or the owner. Casa is feminine, so a man talking about his own house says a casa, and a woman talking about her car says o carro.