You may have come across the term DETERMINERS when browsing through all those grammar books of yours.
What are they? Well, determiners are words (usually short ones) that are used to narrow down the reference of a noun. They are used in front of nouns in English. If the noun is accompanied by another modifier, like for instance an adjective, they are put in front of this modifier instead of directly in front of the noun. For example you say ‘the dog’, but if there’s an adjective, you will say ‘the old dog’. Here you can see the definite article, which is not the only type of determiners, though.
There are quite a lot of different types of determiners. Let’s concentrate on the most important ones.
So, the definite and indefinite articles play this role. The definite article specifies that the referent is assumed to be known to the speaker and the listener. For example:
the car – we know which car we are talking about
The indefinite article narrows down the reference to a single member of a class, like for example:
a car – one element of the class of cars
Then there are quantifiers. Quantifiers specify the number or amount of the entities referred to, for example:
some water, many trees – we understand that there isn’t too much water and that the trees are in a large number
Next, there are demonstrative determiners, which establish the reference by proximity to the speaker and the listener, like here:
this chair – we know that the chair is near the speaker
Then there are possessive determiners. They limit the reference of the noun by establishing a connection with the speaker, the listener or a third party. For example:
your computer – we know the computer belongs to the listener
These are the most important groups of determiners. This classification is not exhaustive, though. There are also wh-words and numerals used as determiners, but this will do for now.