1) Greg and Bob had looked for their dog for two hours before they found it sleeping under a bench in the park. 2) We had been boating in the lake at 10 o'clock yesterday morning. 3) She finished cooking dinner by the time her children returned from school. 4) When Henry opened the door of the living room, his little brother watched cartoons on TV. 5) Sally took a piece of paper and had written her phone number on it. 6) We were driving for six hours before we saw a small hotel beside the road. 7) After my friends left I washed the dishes and switched on the TV set. 8) What had you been doing at the moment the fire broke out? 9) How much was your camera costing a year ago? 10) What were Nelly doing while you were doing the ironing?

Вот как бы я исправил каждое предложение и почему — по ощущениям носителя.

1. Greg and Bob had been looking for their dog for two hours before they found it sleeping under a bench in the park.
   Длительное действие «искали в течение двух часов» до другого прошедшего — это Past Perfect Continuous.

2. We were boating on the lake at 10 o’clock yesterday morning.
   Конкретный момент в прошлом → Past Continuous. С лодкой естественнее on the lake, не in.

3. She had finished cooking dinner by the time her children returned from school.
   К моменту «когда дети вернулись» действие уже завершилось → Past Perfect.

4. When Henry opened the living-room door, his little brother was watching cartoons on TV.
   Фон в момент другого действия → Past Continuous (was watching).

5. Sally took a piece of paper and wrote her phone number on it.
   Последовательные, короткие завершённые действия в прошлом → Past Simple + Past Simple; Past Perfect здесь не нужен.

6. We had been driving for six hours before we saw a small hotel beside the road.
   Длительность до другого прошедшего → Past Perfect Continuous.

7. After my friends had left, I washed the dishes and switched on the TV.
   Можно и Past Simple после after, но классически подчёркиваем предшествование с Past Perfect.

8. What were you doing at the moment the fire broke out?
   Конкретный момент в прошлом → Past Continuous, не Past Perfect Continuous.

9. How much did your camera cost a year ago?
   Глагол cost стативный; о цене спрашивают в Past Simple, не в Continuous.

10. What was Nelly doing while you were ironing?
    Согласование подлежащего (was Nelly), и обычно говорят просто were ironing без the.