Question

Which indefinite article is used before a letter? How do you decide? Is the way a letter is "spelled" the deciding factor?

Answer 1

Use "an" before a word starting with a vowel sound; otherwise use "a".

Explanation:

Usually this means that you use "an" if the word following starts with a vowel;
for example:
`color(white)("XXX")`an apple
`color(white)("XXX")`an eagle
`color(white)("XXX")`an innocent glance`color(white)("XXX")`notice that it is the next word and not the next noun that matters

There are exceptions, most commonly when words start with the letter "H";
for example:
`color(white)("XXX")`an honor
but note:
`color(white)("XXX")`a horse
`color(white)("XXXXXX")`The difference here is whether the consonant "h" is silent or sounded.

"spelling" is a contributing factor in whether a word starts with a vowel sound, but as can been seen (above) it is not the defining characteristic.