In the English language there are two types of articles: definite (the) and indefinite (a and an).
Indefinite articles are used to denote singular number nouns where identity of the person or thing is not established.
'an' is the indefinite articles used before words starting with a,e,i,o or u. 'a' is used before all other words.
'The' the definite article is used before nouns that refer to particular thing, person, animal or place or noun which is already mentioned in the essay, article or a sentence.
Example: I lost the pen, that I purchased yesterday in the shop along with you.
I implemented the suggestion, you had given me.
The is used before a single noun when it is used to represent a whole class.
The dog is a friendly animal.
The lion is the king of forest.
The is used before the names of seas, rivers, oceans, gulfs, groups of islands (but not used before single island) and mountain ranges.
The is also used before the names of holy books, newspapers, magazines, ships, trains and well known buildings.
The is used before directions and unique objects.
The is used before superlative degree terms. Sometimes it is used before comparative degree term.
Ordinal numbers are preceded by the.
The used with people of a particular country and community. But it is not used before languages.