The indefinite article

The indefinite article a/an is used with countable nouns. The 'a' form is used with nouns ending in consonant while the 'an' form is used with nouns ending in a vowel (a,e,i,o,u)
As the name already gives us a hint, this article will turn the noun indefinite.
For example if we say:
I want a book - 'a book' could be any book ever published in this world. The article depersonalizes the noun.
Another example:
I want a good teacher to help me learn English - 'a good teacher'refers to any teacher who has the quality of being 'good'.
What if we say:

I want the good teacher I saw yesterday  help me learn English - in this case, the article  individualizes our choice, it refers to one, unique teacher.

Let's take these two diagrams:

The 'the good teacher' diagram contains only one element wile 'a good teacher' is represented by many X because there are many teachers who are good


As I said before,we can only use it with countable nouns-those nouns which can be counted
For example:
one book-two books
one cat-two cats

Uncountable nouns such as mass nouns, liquids...cannot be used with a/an:

*I want a water
*I want a gold.

Of course you will try to contradict me by saying that it is ok to say 'I want a beer/a tea/a coffe' and I will tell you WHY these structures are ok:
In the case of liquids and mass nouns (gold,meat,flour) we know they do not have any form. Beer,water,juice do not have a form like for example 'man/woman/pencil'. They take the form of the recipients where they are put.
For example :
-if you put coffee in a cup ,it will take the shape of the container and then we can individualize cup as now it is ' a cup of coffe'. Because of this, we can count it : one cup of coffe,two cups of coffe.
In the everyday use, the container is left out so instead of saying:
a cup of tea we say a tea,
instead of a bottle of beer we say a beer.
Do not forget that in such case we are able to count them because we have a container .
We cannot count *one water, *two waters but we can say : 'one bottle of water, two bottles of water'.

More details about uncountable nouns and a list of quantifiers will be available in the 'Noun' chapter.

Let's come back to our discussion.

The indefinite article refers to the kind of a thing, a representative of the species or of the kind.

A cat is a mammal.
A flower is a plant.

Here you can see in the diagram another case when we use the indefinite article:

As you can see, there is only one cat I bought my mother - 'the cat' but 'a cat' is a feline by definition,it represents the species.Any cat is a feline. The diagram contains all the animals we call 'cat' because if the characteristic applies to one representative , it will apply to the rest of the species.
Of course, there is no plural indefinite article so :

Cats are animals.
Flowers are plants.

Questions?Doubts? Leave me a comment and I will reply