Today's grammar lesson

The thing about indefinite articles is they refer to something nonspecific. So choosing "a" or "an" depends on the sound of the word it precedes. "A" goes before words with a consonant sound, no matter how the word is spelled; e.g., "a hard-core porno," "a eulogy," "a historic document," "a yam." "An" goes with words beginning with a vowel sound, such as "an oasis," "an anal stimulator," and "an hour ago."

Many get confused with words starting with the letter h. When it comes to "a" and "an," it depends on if the word begins with a pronounced h ("a history book") or not ("an heir").

When it comes to abbreviations, numerals, symbols, acronyms, or initialisms, "a" and "an" depend on how the term is pronounced; e.g., "a Fox anchor," "an MTV show," "a 900 number," "an & sign."

It gets a further trickier with acronyms and initialisms. With these, the article depends on how the abbreviation is read aloud and how it is used. Acronyms are pronounced as words, where as initialisms are read as letters. Acronyms don't get articles unless they are being used as a modifier (e.g., "the call went to NASA," "a NASA shuttle," "I have AIDS," "the AIDS patients"). Initialisms always get an article (e.g., "an HIV test," "a PSP console," "the ATM").

Of course, this gets even more confusing with networks. While you would say "an NBC news anchor" or "a CBS show," you would not say "I'm watching the NBC." However, it's become proper (leave it to the Brits) to say "I'm watching the BBC." But this all depends on who's doing the editing. Blah. Fuck TV.

(Source: Sections 5.73, 7.46, 8.196, 15.9, Chicago Manual of Style, 15th edition.)