Ответ:
1. The salesman (was helping) the customer when the thief (came) into the store.
2. He (came) to the office at eight, (typed) some unimportant letters, then ( asked) the secretary to bring the managers’ reports and (was analysing) them the whole day.
3. As I (was explaining) the reason for doing this, he (interrupted) me very rudely.
4. The boy constantly (was playing) tricks on his poor mother.
5. He (entered) the door. The candles (were burning) in the corners and the smell of cigarettes (filled) the room.
6. I (was walking) along the street when suddenly I (heard) a woman’s cry.
7. I (was cooking) the whole evening yesterday.
8. What were you (doing) when the earthquake (started)?
9. When I (walked) into the office, several people were busily (typing), some (were talking) on the phones.
10. I (did not like) them because they always (were complaining).
11. We (were trying) to open the door while the firemen (was fitting) the ladder to get into the flat through the window.
12. He was constantly (talking). He (annoyed) everyone.
13. This morning (was) really beautiful. The sun (was shining), the birds (were singing), and everyone in the street (smiled) and (said) hello to each other.