In front of any noun we usually put a ‘determiner’. Determiners identify or 'quantify' the noun. They can be specific (e.g. the man, your keys) or they can be general (e.g. an elephant, any computer, some penguins etc).
Definite Article - the
Possessive Adjectives - my / your / his / her / our / their
Demonstratives - this (este) / that (aquele) / these (estes) / those (aqueles)
Indefinite Article - a / an
Other General Determiners - all (todos) / another (mais um) / any (qualquer) / both (ambos) / each (cada) / either / enough (suficiente) / every (todo) / few (poucos) / little (pouco) / many (muitos) / more (mais) / most (a maioria) / much (muito) / neither / no (nenhum) / other (outro/s) / several (tipicamente 3-5) / some (alguns)
The two situations where we do NOT normally use a determiner are:
Proper names (e.g. I spoke to Tom Jones last night)
Where we are talking about the noun in general or categories (e.g. Richard is studying psychology at university)
I bought a car. (any car, not defined - we normally use a/an the first time we mention something.
The car I bought is blue. (specific car, defined in the previous sentence.
I like blue cars. (I like blue cars in general - therefore we don't need a determiner.
I like blue cheese, too. (again I'm talking about cheese in general, so no determiner. But because cheese is a non-countable noun, we don't add an 's' to the word cheese - see countable/non-countable))
The indefinite article “an” precedes nouns that begin with a vowel sound.
Some general thoughts on Determiners
Note the difference between other and another.
e.g. I saw two men approaching. One was my friend, Tom, but I didn't recognize the other. (o outro)
e.g. I ate a cupcake. It was so good that I ate another (mais um).
But we can also say -
It was so good that I ate another three!
Note how we use either/or (ou X ou Y) and neither/nor (nem X nem Y).
e.g. I will either go to the beach or visit my grandmother.
e.g. I will neither go to the beach nor visit my grandmother. I will stay home instead.