When should you use a and an?

Answer 1

An is generally used when the next word begins with a vowel [A E I O U]. While a is used for consonants

Examples for an:

That is an alligator.

I am an extra in that movie.

etc.

Examples for a:

This is a home.

Where is there a bathroom?

Abbreviations have a somewhat different rule, however!

For "A, E, I, and O", the rule sticks, but for "U", you should use "a" instead of "an". Also, some consonants will require "an" instead of "a", like "S", for example. When it comes to abbreviations, you don't use the first letter, you use the sound of the first letter. For "U", the sound is "yew", and "y" is a consonant sound, so it gets an "a" rather than the usual "an". With "S, the sound is actually "es", and since it's an "e" sound, you use "an" instead.

Examples for an:

He drives an SUV.

She's an HSN addict.

An NFL poster would go nicely on my wall.

The doctor administered an EKG on my wife.

Examples for a:

That guy is a UPS delivery person.

He ran into a DHL truck yesterday.

The doctors said he was a "DOA", unfortunately.

The UPS man will face a "J.U.D.G.E." in front of a "C.O.U.R.T.", and justice will be served.

Okay, so the last two aren't quite abbreviations, but sometimes people do that with words, and the abbreviation rules apply for that too.