A, an, and the are special adjectives. These are called Articles.
When articles combine with nouns, they form noun phrases: There are two articles in English
|The Indefinite article||A or An||A king, an elephant|
|The Definite Article||The||Call the man|
Use Of A / An / The
1. ‘A’ / ‘An’ means one. So we can use ‘a’ / ‘an’ only with singular countable nouns.
2. An is not used with plural nouns or proper nouns.
3. ‘A’ / ‘An’ cannot be used with uncountable nouns.
For example, We cannot say : a milk, a water, a tea, a sugar, a rice.
‘The’ can be used with all nouns (singular or plural).