The difference between ‘a/an’ and ‘one’

The words a/an and one are confusing for many learners of English, especially for learners who do not have articles in their native language. This confusion often causes unnatural sentences like ‘I would like one coffee, please.’ In this post I’m going to give you a simple explanation to help you understand the difference and use these words correctly.

a/an

In English grammar, a/an is known as the indefinite article. It is generally used before singular countable nouns, when we mention them for the first time, and when the person listening does not know which particular thing we are referring to.

  I met an interesting girl at the party last night.
  I’ll be there in a minute. Please wait for me.
  I’d like a coffee, please.

We use an before words that begin with a vowel sound (a-, e-, i-, o-, u-), for example an apple, an orange. Before consonant sounds, we use a (a car, a dog, a grape, etc.)

one

one is a number. It emphasises one of something, not two, three, etc.

  I only met one girl at the party.
  I need one minute, not two.
  “How many coffees would you like?” “Just one, please.”

If a native speaker says “I would like one coffee, please”, the listener will understand this as “I would like not more than one coffee”.

Sometimes, there is very little difference in meaning, for example:

  I only have a dollar.
  I only have one dollar.

In the above example, there is only a small difference is emphasis. The meaning and context is the same, but in the second sentence, the speaker is emphasising not more than one dollar, whereas in the first sentence, the speaker is emphasising that he/she doesn’t have enough cash.