Robin Maben via Flickr
We use A and AN in some fixed expressions to mean "one".
We always use "a" or "an" to mean "one" in fixed expressions of time:
- half an hour;
- a quarter of an hour;
- twice a week;
- once a month; and
- a day.
We also use "a" in some number expressions:
- a few;
- a little;
- a lot of; and
- a number of.
When we use the phrase "a ___ of ___", we never use "one":
- a friend of mine; or
- a customer of ours. etc.
