articles and demonstrative pronouns

Which sentence is correct?

• Her dog has got an ball.

• Her dog has got a ball.

Which sentence is correct?

• I like to write with an pen.

• I like to write with a pen.

Which sentence is correct?

• He has got an unicorn on his sweater.

• He has got a unicorn on his sweater.

Which sentence is correct?

• Could I borrow a calculator for mathematics?

• Could I borrow an calculator for mathematics?

Which sentence is correct?

• Oh no, it's raining! Have you got a umbrella?

• Oh no, it's raining! Have you got an umbrella?

Which sentence is correct?

• He always wears a yellow T-shirt.

• He always wears an yellow T-shirt.

Which sentence is correct?

• A apple a day keeps the doctor away!

• An apple a day keeps the doctor away!

Which sentence is correct?

• The baby is playing with a teddybear.

• The baby is playing with an teddybear.

Which sentence is correct?

• My brother studies at an university.

• My brother studies at a university.

Which sentence is correct?

• There is a binder on the desk.

• There are a binder on the desk.