When to use “an”
When to use a or an? a horse, an apple. A SUV or an SUV?
The rule is to use the article a before words beginning with a consonant sound and an before one beginning with a vowel sound:
Disagreement exists with certain “h” words. For example, not everyone pronounces the h in herb, homage, and humble, but more and more native speakers do.
Depending upon vocal stress, one might hear either “a historic occasion” or “an historic occasion.” Most American speakers would probably say “a historic.” Either is correct in writing.
About the only common English “h words” that definitely require an are heir/heiress/heirloom, honest, honor/honorable, and hour:
an heir to the throne
an honest man
an honorable woman
an hour before
Which indefinite article to use before an abbreviation, a numeral, or a symbol, depends upon pronunciation.
Some examples from the Chicago Manual of Stylee:
an NBC anchor
a CBS anchor
an @ sign
CMOS also points out two possible readings of MS:
an MS treatment
Here the letters stand for “multiple sclerosis” and are read as “Em S.”
a MS in the National library
Here the letters stand for “manuscript” and are customarily read as “manuscript.”
You’d write “an SUV” because SUV is pronounced one letter at a time and the sound of S is “ess.”