There are three general functions of the Article: morphological, syntactic and semantic.
1). Morphological function of the Article.
Article is the main formal material morphological index of the Noun.
For example:
*Would you sponge the water from the table? (water is the noun: it is defined by the article which is a formal index of the Noun);
*Would you water the flowers? (water is the verb: it follows after the personal pronoun in the function of the Subject (direct word order in English))
2). Syntactic function of the Article.
It is called the function of the index of a group of the Noun’s left limit: Article forms a left limit to the following after it group of words that define or attribute the Noun which takes the limit right position. Article defines the Noun but not obligatory is put directly before the Noun.
For example:
A house; A big house; A big stone house; A comfortable big stone house, etc.
3). Semantic function of the Article.
Semantically the Article can express:
a) a certain identification (a concrete or unique thing);
b) a reference to a class of homogeneous or similar things (any of the class).
A. The Meaning of identification is the main for the Definite Article the.
The can mainly be used:
a) in the repeated nomination of an object:
For example:
A day was terrific! I will never forget the day.
b) with the limiting attribute:
For example:
The man, who entered, was really nice.
c) when it is stipulated by the situation:
For example:
Not a word was spoken in the parlor.
d) to define unique phenomena:
For example:
The sun, the moon, the sky, the earth, etc.
e) to express the meaning of the whole class of things;
For example:
The dog is a domestic animal.
B. The reference to a class of homogeneous or similar things (any of the class) creates the Grammatical Meaning of the Indefinite Article (a/an).
A/an can mainly be used:
a) to present a definite object which is not distinguished from the class of homogeneous or similar ones; presents a thing as one from a class:
For example:
You can read a book while waiting.
He is going to be a doctor.
b) to express a generalized meaning which is realized in sentences expressing abstract classification:
For example:
A swarm (any) is more beautiful than a goose (any).