1) They were eating a cake.

Were they eating a cake?

Were they eating a cake or a hamburger? 

They were eating a cake, weren't they?

What were we eating?

Who was eating a cake?

2) He is talking.

Is he talking?

Is he talking or reading?

He is talking, isn't he?

What is he doing?

Who is talking?

3) He has written a letter.

Has he written a letter?

Has he written a letter or a story?

He has written a letter, hasn't he?

What has he done?

Who has written a letter?