1.discovered
2.sat, had seen
3.went, had
4.rang, had
5.waited
6.studied
7.have studied, called
8.slept
9.slept, had rung
10.played , had arrived
11.fell, got
12.walked, have met
13.fell , hurt, rode
14.drove, had happened
15.left, had calle
16.have you done, called
17.had began, ended
18.wrote, rang, had gone , heard , still rang,walked,opened, it has stopped.
19.left
20.have gone
21.had written
22.has joined, was
23.saw, drank
