mg podaje 'when we were on top', co sie rowna temu 'we reached the mountain/we were able to climb it quite quickly'
czyli aby zachowac spojnosc i jednoznacznosc, wszystko, co sie wydarzylo przed 'byciem na gorze, co nam z latwoscia przyszlo' nalezaloby uzyc past perfect
a tak wygladaloby to w chronologicznej kolejnosci:
1.
The day started off misty.
2.
The sun appeared.
3.
We reached the mountain. (to jest punkt odniesienia do uzycia past perfect w zdaniu #2)
4.
We were able to climb it quite quickly. (to jest punkt odniesienia do uzycia past perfect w zdaniu #3, a tym samym i zdania #2)
gdyby pominac zdanie #4 mozna by swobodnie napisac:
The day started off misty, but by the time we reached (=punkt odniesienia) the mountain the sun had appeared. (zachowujac jeden past perfect), ale ze w oryginale jest to zdanie punktowe #4 dodane, to i drugi past perfect jest zachowany
The day started off misty, but by the time we had reached the mountain the sun had appeared and we were able to climb it quite quickly (=nowy punkt odniesienia).