I think that that's nothing better than illustrate what we are talking about here with some examples.
Um amigo - a male friend.
Uma amiga - a female friend.
Umas amigas - some female friends.
Uns amigos - some male (or male and female) friends.
In this last example I write " some male (or male and female) friends" because in Portuguese, when you have to refer to a group of mix gender - males and females together - you use the plural masculine form.
So, if you said to me that you are going out with "uns amigos", I'd understand that you are going out either only with some male friends or with some male and female friends in a mixed group.
Some more examples:
Um livro [oong lee-vroo] - a book.
Uns livros [oongsh lee-vroosh] - some books.
Uma ideia [oomah ee-day-eeah] - an idea.
Umas ideias [oomash ee-day-eeash] - some ideas.
Uma caneta [oomah cah-nay-tah] - a pen.
Umas canetas [oomash cah-nay-tash] - some pens.
And so on.
You see? It's not difficult at all! I'm sure you understand it now!
So, what you have to do next is practise these indefinite articles with every single noun I give you here and replace the definite article by a correct indefinite article (e.g. instead of saying "a água" you should say "uma água", instead of saying "a ajuda, you must say "uma ajuda") and so on.
Hey! have you started???
Come on! Practise your vocabulary now!
Boa sorte! (Good luck!)
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