Grammar question?

No, it's not right to add "a" unless "Portuguese" is an adjective, "I am Portuguese", but "I am a Portuguese man". You can use "a(n)" with a continent name: "I am an American", "I am an African", "I am a European", but the "a(n)" is optional: "I am American", "I am Asian", "I am European". But with most nationalities, you cannot use "a(n)". There are a very small number of exceptions where there is a different word: "I am a Swede", but "I am Swedish"; "I am a Jew", "I am Jewish"; "I am a Spaniard", "I am Spanish"; "I am a Slav", "I am Slavic"; "I am a Pole", "I am Polish". There is probably a rule here, but I just had my Thanksgiving feast and I'm a little drowsy. ;)