A vs An. Using articles in English

A  VS  AN

Rule 1:   A Rule 2:  AN
The article A is used before singular, countable nouns that begin with consonant sounds

Mark visited a church.

She bought a table.

A rabbit is white.

*Note

There is a university here.

The article AN is used before singular, countable nouns which begin with open vowel sounds.

   

He is an actor.

She didn’t get an invitation.

I saw an eagle at the zoo.

It takes an hour to get here

Rule 3 : A(AN) USE 4: A(AN)
A(AN) means “one” or “a single”. You cannot use A(AN) with plural nouns.
I saw a bears in Yellowstone National Park. Not Correct  

I saw bears in Yellowstone National Park. Correct

A(AN) should agree with the first sound in the adjective or the adverb-adjective combination.
He is an excellent teacher.  

I saw a really beautiful eagle at the zoo