How we change what others think, feel, believe and do
Definite and indefinite articles
Use 'the' to specify a single item and to indicate that there is no other item like this.
Use 'a' to create a general instance of an item, implying that there may be many other similar items.
Add emphasis to the words to create further attention to the singularity or generality of what is being said.
In arguments, you can deliberately move between the definite and indefinite article to point out that something is unique when it is being treated as general or not unique when the definite article is being used by the other person.
This is the best day of the Summer.
This is a good day.
This is not the best day, but it is a good day.
The definite article (the)
The word 'the' is the definite article, because it refers to a defined, single noun. An unspoken effect is to create uniqueness, excluding the possibility of any other item that is the same as this.
The indefinite article (a)
The word 'a' is the indefinite article, because it refers to an instance of a noun. By implication, there is at least one other (and possibly many more) items like this. How similar the items are depends on the level of definition of the noun. Thus 'a dog' gives a very broad scope for what other dogs may look like, whilst 'a chocolate labrador bitch' reduces this scope.
The definite and indefinite articles are adjectives, as they describe nouns. They are also determiners as they determine quantity.