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?