Countable nouns refer to people, places, or things that can be counted:
NOTE: A is used before a consonant sound: a boat, a shiny bus. An is used
before a vowel sound: an elephant, an intelligent old man.
When not to use a (or an). A (or an) is not used to mark non countable nouns.
Non countable nouns refer to things that cannot be counted: water, oil, money,
laughter, patience.

A or an can be used to talk about things in general. Choose either A or AN to fill in the sentences.

He had to go and see … doctor.
There was … extremely annoying man outside.
He was in no mood for … examination.
There was … letter waiting for him.
I think the dog ran after … car.
What is …nice girl like you doing here?
Is there … easier way to do this?
What …. nice man he is very generous.
Have you had … club sandwich before?
Sit down and I will tell you … story.

Listen to this story about a man who witnesses an accident fill in the gaps with the correct article.

Jim was driving his car to work on the motorway when he saw … accident. … car in front of him had skidded
off the road and crashed into … hedge. Luckily … ambulance had already been called and was there at the
scene. Jim was forced to stop his car as … policeman was making all the cars stop. Jim wondered how late
he would be for work, he had … important meeting that morning and didn’t want to miss it!

To make a general statement about something we can use a/an article, or change it to the plural.

e.g
A mouse is a small rodent.  A television can be bought for quite cheap.
Mice are small rodents.  Televisions can be bought quite cheaply.