Not all languages use articles as often as English does. If you ran a computer program that sorted and counted every word in this book, you’d be on the fast track for membership in the Get-a-Life Club. You'd also find that articles, a branch on the adjective family tree, are the most common words, even though the article-branch includes only a, an, and the. But be aware, these articles can alter the meaning of a sentence.
Melanie wants the answer, and you’d better be quick about it.
This statement means that Melanie is stuck on problem 12, and her mother won’t let her go out until her homework is finished. A really good movie is playing at the cineplex, and now she’s on the phone, demanding the answer to number 12.
Melanie wants an answer, and you’d better be quick about it.
This statement means that Melanie simply has to have a date for the prom. She asked you a week ago, but if you’re not going to be her escort, she’ll ask someone else. She’s lost patience, and she doesn’t even care anymore whether you go or not. She just wants an answer.
To sum up: Use the when you’re speaking specifically and an or a when you’re speaking more generally.
A apple? An book? A precedes words that begin with consonant sounds (all the letters except a, e, i, o, and u). An precedes words beginning with the vowel sounds a, e, i, and o. The letter u is a special case. If the word sounds like you, choose a. If the word sounds like someone kicked you in the stomach — uh — choose an. Another special case is the letter h. If the word starts with a hard h sound, as in horse, choose a. If the word starts with a silent letter h, as in herb, choose an. Here are some examples:
An aardvark (a = vowel)
A belly (b = consonant)
An egg (e = vowel)
A UFO (U sounds like you)
An unidentified flying object (u sounds like uh)
A helmet (hard h)
An hour (silent h)
Special note: Sticklers-for-rules say an historic event. Everyone else says a historic event.