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When to use A/AN not ONE

A close up of a clock
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;
  • little;
  • a lot of; and
  • number of.

When we use the phrase "a ___ of ___", we never use "one":

  • friend of mine; or
  • a customer of ours. etc.
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