Definite articles and preposition contractions
Every Portuguese noun is either masculine or feminine, and the article matches: o for masculine, a for feminine. The plural forms add an -s: os, as. The trickier piece — and the reason a beginner book of Portuguese looks so different from Spanish — is that prepositions fuse with the article. de + o isn't 'de o', it's do. You can't avoid these contractions; they're how Portuguese is actually spoken and written.
The four definite articles
o = the (masc. sing.): o gato, o pão, o livro. a = the (fem. sing.): a casa, a água, a mesa. os = the (masc. plural and mixed groups): os gatos. as = the (fem. plural): as casas. Use these whenever you mean a specific, known thing — the same situations English uses 'the'.
Articles before names — a Portugal habit
In European Portuguese it's normal — even friendly — to put an article before someone's first name: A Ana é portuguesa, O Pedro está em casa. In Brazilian Portuguese this is much less common; people just say Ana é portuguesa. Either is correct, but if you're reading a Lisbon novel and see a Maria, that's not a typo.
de + article → do, da, dos, das
The preposition de ('of' / 'from') always fuses with the following article. de + o = do; de + a = da; de + os = dos; de + as = das. A casa do Pedro = Pedro's house. A mãe da Ana = Ana's mother. O livro dos amigos = the friends' book. You'll never write de o or de a — it's always one word.
em + article → no, na, nos, nas
Same fusion with em ('in' / 'at' / 'on'). em + o = no; em + a = na; em + os = nos; em + as = nas. Estou no café = I'm at the café. Estou na casa = I'm in the house. (Note: em casa without the article is the idiom for 'at home' — like English drops 'the'.) As crianças estão nas escolas = the kids are at the schools.
a + article → ao, à, aos, às
The preposition a (meaning 'to' for direction) also fuses. a + o = ao; a + a = à (with a grave accent — that's how you tell it apart from the bare article a). a + os = aos; a + as = às. Vou ao café = I'm going to the café. Vou à casa da Ana = I'm going to Ana's house. The grave accent on à is called the crase and it's not optional.
