A/an is used with singular nouns to introduce something new that the listener or reader does not know about.
I saw an accident. I have a new bag.
I have an older sister.
A/an is used in expressions of time and quantity.
Forty times an hour Thirty pence a kilo.
Rules for using the The is used when the listener or reader knows or will soon know what we are talking about. It tells the listener or reader to think about things they have already or seen. The is used with a noun to show:
1.The noun refers to something that has been said or written.