"Meu" or "O meu"
"My xx" is sometimes translated to "O meu/A minha" and sometimes to "Meu/Minha". When do I need to use the "O"/"A" before the "Meu"/"Minha"?
when it is a possessive adjective, you can choose wheter or not to use the article. when it works as a pronoun, you must use the article. examples: my car is better than yours = (o) meu carro é melhor que O seu. on the 2nd example, you have to use the article.
In Portugal the article is always present. So, my train in Brazil is "Meu trem" but in Portugal it's "O meu comboio" (Comboio é a palavra portuguesa para trem)
You can't say "meu trem" in Portuguese, even writers and important people in Brazil get to manage the language properly. "Meu trem" is quite informal, actually.
Meu (without the article) means "mine" NOT "my", so there's a direct translation into English, but "o meu" also works as a pronoun.
As Pauloenrique said, when it is a possessive, the name follows, so you can ommit, but you must use the article when it's a pronoun (in replacement of a substantive). The brasilians tend to ommit the pronoun. The portuguese use it more. Sometimes it doesn't make much of a difference. For example: pt-br) Deixei meu carro no acostamento pt-pt) Deixei o meu carro na berma It means "I left my car on the hard shoulder" (or "I left my car at the roadside") The use (or not) of the pronouns is perfectly understood by a brasilian or a portuguese (as for the word "acostamento" it is not used in Portugal and people will have difficulty understanding).
But there are some subtle differences (for both versions of portuguese). 1) Ela é minha amiga. 2) Ela é a minha amiga. On the first case it means that "she is a friend". On the second it means that she is a specific friend that you previously talked about. It could also suggest that this woman is the only friend that you have.
Portuguese is spoken in many countries, "Minha amiga" is only heard in Brazil, but it's quite informal. In the end, even writers and important (educated people) say "A minha amiga".
What if I have several cars (in fact, I have none)? Should I use "um meu carro"?
Basically, if the possessive is followed by a noun (meu carro, minha amiga), the article can be omitted.
I just wanted to add that in European Portuguese, it will always be "o meu" or "a minha."
It's not in European Portuguese, but in STANDARD Portuguese. Don't forget Portuguese is spoken in many countries, and in all of them you'll just hear "O meu carro", not "meu carro", except in Brazil, obviously.
Agreed. I don't remember how it was in Portugal, but In Brazil we definitely accept both, sometimes with slightly different meanings.
You should use the article all the time, that's the way Portuguese works, and that's he way it's spoken in any Lusophone country.
male gender "meu" example: O meu carro, O meu cachorro, O meu pai, O meu irmão, O meu lápis, O meu pente, O meu corpo female gender "minha" example: A minha casa, A minha irmã, A minha mãe, A minha bicicleta, A minha Cadela, A minha caneta, A minha cama
You can omit the "O"/"A" if you are using a noun. For example: O meu nariz. meu nariz.