It was already mentioned in the “past imperfective” lesson that the main usage of the past imperfective is to express habitual actions in the past.
در آن زمان مردم از کسوف میترسیدند.
In that time, people were afraid of eclipse[s].
Also, in the “conditional sentences” lesson, another usage of the past imperfective was introduced; it can be used in conditional sentences in the past both as the condition verb and as the verb that specifies the result of the condition.
اگر حقیقت را میگفت کشته میشد.
If he said the truth, he would be killed.
Apart from these two, there is another important situation where the past imperfective comes into action; in sentences expressing the fact that something had to happen in the past and we do know now whether it has happened or not. Unlike English, the same structure is used for cases where the action has actually happened and where it has not.
باید درس میخواندی.
باید آنجا میبودی و به برادرت کمک میکردی.
خوب بود با پدرت هم مشورت میکردی.
باید این کار را میکردم.
باید منظور ما را میفهمید.
1- You should have studied. 2- You had to study. (As you actually did.)
1- You should have been there and helped your brother. 2- You had to be there and help your brother.
It would be good if you consulted with your father too.
1- I should have done this job. 2- I had to do this job. (and therefore I did it.)
1- He should have understood our intention. 2- He had to understand our intention.
Don’t confuse this usage with that of the past subjunctive (the third usage of the past subjunctive in our list; necessity), which was for cases where the action is expected to have happened in the past but we don’t know whether it has actually happened or not.