21. Has improved - moved
-> HTHT since QKD
22. Are waiting - have been waiting
-> HTTD: S am/are/is Ving 
S have/has been Ving for …
IV
1. The bill must be paid at once
-> S must be VPII by O.
2. English is spoken by many people in the world
-> S am/are/is VPII by O.
3. It’s a difficult question to answer
-> It’s a/an adj N to Vnt
4. Your bicycle will be repaired by me tomorrow afternoon 
-> S will be VPII by O.
5. Linda is interested in watching cartoons with her little brother
-> be interested in N/Ving
6. Many dangerous diseases can be cured by penicillin
-> S am/are/is VPII by O.
7. Is rice exported to Britain by your country?
-> Am/Are/Is S VPII by O?
8. I was very delighted that I passed the final exams
-> S tobe adj that S V…
9. It’s very important to keep the environment clean
-> It’s adj to Vnt
10. The time machine will be shown to the public when it is finished
-> S will be VPII by O.