Articles are short words which come before nouns to show whether they refer to a general or a specific object.
The indefinite article (a) or (an) is used to talk about something in general.
I work in a school.
(an) is used instead of (a) before words that start with a vowel
I work in an office.
The indefinite article is used to talk about a general thing when the exact one is not yet known.
I want to buy a car.
When a person or thing has been mentioned already the definite article (the) is used the next time the person or thing is mentioned.
The car I bought is great.
The definite article (the) is used to talk about a specific person or thing that everyone understands.
I love this restaurant. The staff are great.
The definite article (the) is used before superlatives.
This is the best pizza in Sydney.
The definite article (the) is used when there is only one particular person or thing.
The sun was shining.