How can we describe some cases, in which an act occurs during some process? Can we use -았고/-었고/-였고 + 있다 or -고 + 있었다?
When I entered, he was jumping on the floor.
To describe a continuous act in the past, we can combine -고 + 있었다.
내가 들어갔을 때, 그는 바닥에 뛰고 있었어요.
In general, when we have a chain of verbs, tense markers like 았/었 and 겠 will usually go on the outermost (i.e. last) of the verbs:
해 주고 싶었어요.
But there are exceptions when it should logically belong to an earlier one:
밥 먹었냐고 물어보세요 (Ask him whether he has eaten; the past applies to the action of eating, not asking).