Article use of A An and The.

An apple a day keeps the doctor away.
An apple ? What colour is it? How is it? It is a ripened or rotten one.
Who knows? So it is indefinite. We can't say it is red [or] yellow [or] green.
We can't say whether it is ripened [or] rotten.So it is indefinite.
So we use indefinite article stating above one apple AN APPLE.
Words beginning with a vowel sounds always in singular goes with,
AN example. An apple An egg. An idea. An axe. An umbrella.
An is not an enemy of an elephant.
For an enemy it is an internal matter.
An M, P. , [or] an M, A., We say because they Sarts with a vowel sound.
An honest man is an honorable one
He waits for an hour before he gets his work done.