Grammar Exercise: A / An or One?

Do you know when to use “a/an” or “one” before the noun? Here are some simple grammar rules for you.

A / An
When we talk about things for the first time, we use the indefinite article. (A sandwich / a glass of water / an apple, etc).
We use “a” before words starting with a consonant sound, and “an” before words starting with a vowel sound.  (Remember: some words start with a vowel but are pronounced with a consonant: university / use, etc.)

We use “one” when we’re talking about the quantity of something.
“He has one degree – not two.”

We also use “one” to suggest that there are other alternatives. For example:
“It’s a good idea” = your idea (and it’s the only idea) is good
“It’s one good idea” = you might have other good ideas as well

We also use “one” to give a precise amount:
“One thousand pounds” = an exact number of one thousand pounds

One and Ones
We can use “one” and “ones” as pronouns.
“Which one would you like?” (= Which one thing of many.)
“Which ones would you like?” (= Which things of many)

A, An, or One

Choose the correct answer.

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