Determiners
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).
Specific Determiners
Definite Article - the
Possessive Adjectives - my / your / his / her / our / their
Demonstratives - this (este) / that (aquele) / these (estes) / those (aqueles)
General Determiners
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)
Examples:
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))
Indefinite Articles
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.