In English a subject is almost always necessary to form a complete, natural sounding sentence. In Portuguese, it seems like one can choose to omit personal pronouns. For example, in the following sentences, the pronoun may be omitted.
[Eu] Vou comer alguma coisa.
[Nós] Moramos em São Paulo.
That's no problem, because everyone knows it's 1st person singular (resp. plural) because of the ending of the verb. But I don't exactly know when (if ever) it's better to include a pronoun. Is the version without pronoun considered informal? Does formal Portuguese always include the pronoun?
I also have a feeling that the first person pronoun is omitted much more often than the third person pronoun.
[Eles] Comem muito.
Is there a reason for that?