When do you use 'a', 'an' and 'one'? Are they interchangeable?

When do you use 'one', and when do you use 'a'? Are they interchangeable?

I get asked this quite often, unsurprisingly, so I'll try to explain it in words here.

'A' is used before numbers and measurements to mean 'one' and is more common in an informal style. However, in more formal language, or when we want to be precise, we use 'one':

   A hundred people came to the conference.

   One hundred people came to the conference.

Both are correct.

   I'd like a coke, please.

   I'd like one coke, please.

Although both are grammatically correct, the former sounds more natural, unless we are trying to be precise: I want just one coke, not two.

   This tablet weighs only a hundred grams!

   This tablet weights only one hundred grams!

The latter carries a stress on 'one' to indicate the lightness of the tablet.

For hundreds and thousands, we usually use 'a' at the beginning, but not in the middle:

   159      a hundred and fifty-nine / one hundred and fifty-nine

   5159    five thousand, a hundred and fifty-nine

              five thousand, one hundred and fifty-nine

However, see these:

   1000    a thousand / one thousand

   1001    a thousand and one / one thousand and one

   1159    a thousand a hundred and fifty-nine

               a thousand, one hundred and fifty-nine

               one thousand, one hundred and fifty-nine

In other words, when it is a thousand and some hundreds, we say 'one thousand', not a thousand.

While I'm on the subject of 'a', let me touch on 'an', too.

Be careful not to make the mistake of using 'an' before all vowels. 'A' is used before a word that begins with a consonant sound, while 'an' is used before a word that begins with a vowel sound.

In the above image (click it to see an enlarged version), you can see how vowel sounds are represented. When in doubt, consult a dictionary. Here are some examples of possibly confusing words:

an MP3 /ˌem piː ˈθriː/ player
a university /ˌjuːnɪˈvɜː(r)səti/
an HIV /ˌeɪtʃ aɪ ˈviː/
a hi-fi /ˈhaɪˌfaɪ/ system
an hour /ˈaʊə(r)/
a hotel /həʊˈtel/
a one-man show /wʌn/

If you are still unclear, post a comment, and I will try to help.

Related posts:

Teaching & Learning Pronunciation
Mobile Pronunciation Application
What is your English?
Pronunciation website
Pronunciation of regular past simple