1.(1) A / an are indefinite articles. Use a / an when you are not talking about something specific:
Do you have a pen?
(2)'The' is the definite article. Use them when you are talking about something specific:
Do you have the pen I gave you last week?
2. In conversation or in writing, when you introduce something for the first time, use a / an:
A snow storm is wreaking havoc across several northeastern states in the United States.
When you talk about the same thing again, use the. It is used when the speaker and the listener know exactly which thing you are talking about:
The storm has forced several airports to close and stranded travellers.
In the first sentence, I introduced the topic: a snow storm. After I mentioned it in the first sentence, everyone knows which snow storm I am talking about.In the second sentence I can say the storm because now everyone knows which storm I am talking about: the snow storm that is affecting the northeastern U.S.