Ответ:
1. Nikita isn't riding his bike.
2. Twinky isn't riding his fly-bike.
3. The children aren't playing volleyball.
4. Twinky isn't playing starball.
5. Now you are writing sentences.
6. Are her parents listening to music?
7. The girl isn't doing her homework now.
Объяснение:
5. Наречия once, twice, yet, still, now, soon и др., выражающие время и повторение, обычно стоят или в самом начале, или в самом конце предложения.