1/ had been working (here you specify the order of events by using Past Perfect)
2/ didn’t understand (it’s a finished activity, it finished when the problem has explained)
3/ had been trying / was trying, didn’t succeed (generally, you can use both if you want to, and it depends on what you want to highlight – the progressive aspect gives more prominence to the duration of an event)
4/ had been knocking / was knocking (here it depends whether you want to signify the order of events or the duration – later: opened
5/ has been learning (this is an uncompleted action from the part progressing to now), is learning (the duration) or learns (the state)
6/ hasn’t read (prototypical Present Perfect case with yet)
7/ had been in love (we report something from the past)
8/ had been waiting (reporting from the past)
9/ has been reading (the progressive aspect, unfinished), hasn’t finished
10/ were having (a background activity)
wrzucam też pozostałe, bo chyba pytanie zostało usunięte :)
11/ had been mending (a background activity in the past), came (a short activity in the past)
12/ Had he been (more prominence to progression of the activity, not to the activity itself), fell (a singular activity from the past)
13/ was painting (and we are not sure / we are not interested in whether it’s finished or not)
14/ had been telling lies (a longer action from the past), managed (a shorter action from the past)
15/ did not invite (typical Past Simple)
16/ had had (3rd conditional)
17/ will have been flying (typical FPC)
18/ have called (an action in progress, unfinished)
