1. a. They haven't written a letter yet.
b.Have they written a letter yet?
Who has just written a letter?
What have they done?
What have they just written?
They have just written a letter, haven't they?
2. a. The boy has't played with a dog yet.
b. Has the boy played with a dog?
Who has played with a dog?
What has the boy done?
What has the boy played with a dog?
The boy has played with a dog, hasn't he?