We use "a" or "an" when referring to something for the first time or when the specific thing is not important. We use "the" when referring to the same thing again or when something is unique. Some exceptions include not using articles with meals, means of transportation, general concepts, or place names like countries and cities.

1. We use A/n When we don't know or it isn't important which one:

2. eg: He was working in a hardware .

3. With jobs: eg: He is a foreman

4. To talk about a person or thing for the first time:

5. eg: They've got a computer's address book

6. We use The To talk about the same person or thing for the second/third/ fourth time

7. eg: She wrote it in the computer's address book.

8. With superlatives:

9. eg: She's the most beautiful girl in the world