Articles a / an / the

Speaking simply, all the things / nouns which are of same kind, but existed in more quantity and their names starting with a consonant sound must be preceeded by "a"..

a potato         a pen        a car        a mirror         a zebra         a goat        a bunch        a boy          a song             a deer           a kick

all the things which are of same kind but existed in more quantity and their names starting with a vowel sound must be preceeded by "an"....

an umbrella           an empathy              an aeroplane            an event         an apple          an elephant        an ice cream             an owl         an animal

The words like University, Union,  Universe, etc although they are starting with a vowel , we pronounce them with  a consonantal sound... so, we should not keep "an" before...
The words like "Honesty" though they are starting with a consonant, we keep "an" before it... as they are pronounced with a vowel sound in beginning.

The things which are unique in nature, or they exist as only one (single) must be preceeded by THE

The Universe          The Earth              The moon               The sun                The Time

Articles should not be preceeded the names of people and countries..
we should not say The Rama , The India

cases :

a girl sat on a wall  
(here we generalising both the nouns and not giving importance to either girl or wall)

The girl sat on a wall 
(Here we are particularly telling about GIRL and generalising the noun WALL)

a girl sat on the wall 
(here we are generalising the GIRL and talking mainly about the WALL)

The girl sat on the wall 
(Here we are equally giving importance to both GIRL and WALL)