Grammar question: When to use "a" instead of "an"?

I believe (not a grammar specialist) that if the vowel is follwed by an “n,” as in unicorn, you then use “a.”

Edit: scratch that, you do say “an” anaconda.

It’s not the spelling of the word that counts so much as its pronunciation. The beginning sound of “unicorn” is “Y” as in “yes”, which we consider a consonant in most cases, so we use “an”. This works the other way too: we use “an” with “honor” even though it begins with a consonant, but “a” with “herd”, because the beginning sounds are what matters.

It’s if the sound of the first syllable is vowel-based that you use the an. In your example, unicorn sounds like YOO-nee-corn. See, the sound of the word starts with a consonant-sounding Y…

… oh.

What @harp said.

This site, the Purdue University Online Writing Lab, has the rule and exceptions written out clearly and even offers interactive exercises.

Ex. An historic event

Just think of the word phonetically!

Interesting that the Purdue page ignores my pet peeve exception, which chris6137 mentioned:

an historic event

Which drives me crazy when people perpetuate that, since it sounds very wrong to me. It only makes sense if you have a certain accent where you don’t pronounce that H:

an ‘istoric event, gov’na?

Check out aspirated-unaspirated or voiced-voiceless “h’’. It appeared in Classical Greek and was called “soft” or “hard” breathing. It shows up in French and Hindi, and possibly other languages.

Les haricots verts – (string beans) – the “s’ in “Les” is pronounced. True for all French nouns starting with “h,” as far as I know.

When writing, always use “an” with “historic.” “Hostile” – “a hostile” or “an hostile.” “A” sounds better.

I sound things out when I read, so when I read “an historic”, it either sounds just like “an hostile” does, or I imagine the writer saying, “an ‘istoric” with a severe Cockney (?) accent. I guess I have “historic” locked in an aspirated pronunciation.