However, with this rule there is an exception. If you are using a word with a silent 'h', such as 'honorable' or 'honest' then you have to use 'an'. Therefore, it would be "an honorable man" and "an honest mistake"
With words like ‘umbrella’, ‘ice cream’ and ‘apple’ you have to use: “an umbrella”, “an ice cream” and “an Apple
It’s a wide door.
A girl walks to school.
I have a pencil.
I would like a green apple.
He buys a computer.
If you use words and phrases such as 'ball', 'glass of water' or 'cup of coffee', then it would be "a ball", "a glass of water" and "a cup of coffee"
When ‘u’ has the same sound as ‘y’ in “you”, or ‘o’ has the same sound as ‘w’ in “won” then you must use ‘a’.
An example would be "a one-legged man" and "a European trip"
He knows how to fly an airplane.
I have an uncle and an aunt.
There ir an elephant in the park.
I will be home in an hour.
Sarah has an umbrella.