1) When the preposition "de" is together with the definite articles "a" or "o" for a feminine or masculine word, both the preposition and the article combine to form "DA" or "DO".

Examples:

a) Eu sou DE + O Brasil. = Eu sou DO Brasil. (in this case, Brasil is a masculine word)

b) Eu sou DE + A China. = Eu sou DA China. (in this case, China is a feminine word).

c) Eu sou DE Portugal. = Eu sou DE Portugal. (in this case, Portugal is a neutral word, so there is no article)

d) O livro DE + A professora. = O livro DA professora. (in this case, professora is a feminine word)

2) If a noun ends in "a", it's USUALLY feminine.

Examples: a casa, a janela, a porta

3) If a noun ends in "o", it's USUALLY masculine.

Examples: o cachorro, o quadro, o amigo

4) Nouns are usually either feminine or masculine. However, for proper names (cities, countries, etc), there are some neutral nouns.

Examples: Portugal, Israel, Sao Paulo.

Unfortunately, there are many exceptions to the feminine and masculine rules. For this reason, it's really helpful if we learn the article together with new nouns.