Question

6 They have been friends for/since many years.
7 What were you doing/had you done at six o'clock yesterday?

8 By the time I went to the cinema, the film was already starting/had already started.

9 Listen! Someone sings/is singing!

10 Bill watched/was watching TV while I was reading a magazine. ​

Answer 1

6. for. (Они дружили на протяжении скольких-то лет).
7. were you doing. (Длительное действие, поэтому past continuous).
8. had already started. (Действие завершилось и предшествует другому действию в прошлом).
9. is singing. (Поет в данный момент, поэтому present continuous).
10. was watching (два длительных действия)