Correct me if I'm wrong. . .

I know what an is used for, thank you very much.
'An' is instead of 'a'; you use it before words that start with a vowel.
But...
I've seen it used before abbreviations. Like, for an example;
'I got an SAT test to write on.'
or
'Have you had an MRI before?'
Those are just a few examples, though.
So, do you put 'an' in front of an abbreviation, or do you just follow the rule you follow with everything else?

Not sure about the SAT, but with MRI, it you were to spell it how is sounds, it'd be emm-are-iy.

An emm-are-iy.

It starts off with a vowel sounds, so you treat it as though you write a vowel at the beginning of it.

Yes?

(Same with the SAT: ess ay tee. Try saying 'a ess.' )

I don't think that "an SAT" is the common usage thought...generally it's pronounced "sat" so I think "a SAT" would be more appropriate.

I've always just gone by how you say it, and if it's a vowel sound at the start, use "an", otherwise use "a" as for normal words.