-Use a and an with singular countable nouns.
Examples : a heroine, an honest( the h is silent ) man, a unit( pronounced yu instead of u)
-Do not use a or an with pural countable nouns or specific nouns.
Examples : heroines, honest men, units.
-Use the with specific countable nouns
Example : The honest taxi driver was praised by the police.
-Use the with the superlative form of adjective.
Example : the largest, the tallest
-Use the with unique nouns, that is, when there is only one object of its kind.
Example : the sun, the moon, the Earth, the sky.
-Use the with same ……as
Example : Some types of mineral water taste the same as tap water. I have the same pencil case as yours.