Put the verbs in parantheses into the Past Simple or the Past Perfect Continuous:
5. Although he (try) hard to memorize the poem he (cannot) even remember the first stanza.
6. Professor Evans (lecture) on socialist economy for 15 years before he (decide) to change the subject.
8. Samantha (work) as a model for two years before her picture (appear) in "Playboy".
9. We have evidence that Mrs. Reynolds (clean) the windows shortly before she (fall) down.
10. The prisoner (saw) the bars for three nights before he finally (make) his escape.
11. Although the girl only (miss) for eight hours the parents (decide) to call the police.
12. The dog (whine) for the whole day before our neighbour finally (throw) him a bone.
13. The driver (discover) a pirate ship that (lie) on the sea bed for two centuries.
14. Roy (work) over a math problem for two hours before his sister (come) and (help).
15. The rabbit (be) very tired. He (run) from the dog for hours.
17. No wonder the engine (break) down yesterday; it (run) for 24 hours.
18. At last the experimenters (find) the vaccine they (try) to develop for three years.
19. The fresco which the artist (paint) since the previous summer (be) ready in August of 1661.
20. It (be) only last year that Joan (finish) the book she (write) since 1981.