In English language, A, An, and The are called Articles.
“A” and “An” are called indefinite articles as they do not refer to a particular noun while “The” is called a definite article as it refers to a particular noun.
Articles (A, An, The) are not a separate part of speech. They are adjectives because article “The” is a brief form of “That”.
Generally, No article can be placed before material and abstract noun.
“An” is placed before a word beginning with vowel sound while “A” is placed before a word which does not begin with vowel sound.
Before the names of seas, oceans, ships, Provinces, Group of Islands, Books, Magazines, Directions, Heavenly bodies, Newspapers, Canals, Mountain ranges, Nations, Rivers, and before the superlative degree of Adjective, we use article “The”.