• I bought a book yesterday. The book is about rock music.

When we use a noun in a sentence for the first time, we use a or an. But we use the when we use that noun again.

• They gave me an expensive watch, but I didn’t accept the watch.
• There was a boy with yellow t-shirt in the party yesterday. Do you know the name of the boy?

We use the when talking about something known or definite.

• They are playing a computer game.
• They are playing the computer game I bought last week.
• I saw a man in the garden
• I saw the man in the garden

• I know a place where we can have lunch today.
• I know the place you are talking about.