Use of "an" in front of vowels

[thapliyalabhi]

I know we use "an" in front of letters/words based on their sound(i.e. whether their sound starts with a/e/i/o/u).
What I would like to know is whether the rule is different while writing. I mean is it correct to WRITE "an useful prediction", but SAY "a useful prediction", or it should be "a useful prediction" even while writing.

[RonPurewal]

I know we use "an" in front of letters/words based on their sound(i.e. whether their sound starts with a/e/i/o/u).
What I would like to know is whether the rule is different while writing. I mean is it correct to WRITE "an useful prediction", but SAY "a useful prediction", or it should be "a useful prediction" even while writing.

"An useful..." is incorrect aloud AND incorrect in writing.

More importantly"”
The GMAT does not test a/an/the at all. So, this forum is not an appropriate place for questions like this one. It's important for us to keep a tight focus on the actual content of the exam.