Definite and Indefinite articles.

We use a / an with a singular noun when we talk about something for the first time . We use the with a singular or plural noun when we talk about something for the second time:

• There´s a shirt here and a sweater. The sweater´s Rob´s and the shirt´s mine.
• Here are my shirts and sweaters. The shirts are clean but the sweaters are dirty.

We use a /an when we talk about a thing but we do not say which thing it is:

• She works in a shop in London.

We use the when it is clear which thing we mean because there is only one:

• I live near the post office.

We use a / an when we:

- talk about a person´s job: Jenny´s a teacher.

- describe something or someone with the verb be: That´s a good book.

We use the with:

- musical instrumets: Rob plays the piano.

- the names of cinemas, theatres, museums and hotels:

the Picturehouse, the Globe Theatre, the British Museum, the Hilton Hotel

We do not use a / an / the with:

- sports and games: We sometimes play football.

- school subjects: I like Biology.

- meals: When´s dinner?

- the names of streets: Baker Street

- the names of countries, cities and towns: Japan, Rome, Cambridge 

Exercise 1

Exercise 2

For more information about "Indefinite articles" watch the video below.