⭕️ Use one when the number is important; when you want to emphasize that it is only
one (and not two or three or more
▶️ One of these eggs is rotten, but the others are OK.
▶️ I wanted to buy three CDs, but I didn’t have enough money, so I bought only
one.
In all other cases, when the fact of being “one” is not important, use a / an:
▶️ I had an omelet for breakfast.
▶️ I bought a new CD yesterday.
What about the difference between a and an? We use an before words beginning
with a vowel sound, and a before all other words:
▶️ an apple / a banana
▶️ an ice cream cone / a piece of cake
▶️ an egg / a carrot
▶️ an omelet / a steak
▶️ an umbrella / a uniform
(because “uniform” is pronounced yuniform)
▶️ an hour / a hat
(we use an with hour because the H in hour is silent, but the H in hat is not)