USING THE SEQUENCE OF TENSES
Start with the main verb. Find it and assign it to one of two groups: if the main verb is present or future, then it belongs in primary sequence; if the main verb is in the past, then it belongs in secondary sequence. Now turn your attention to the subordinate clause(s). If we’re in primary sequence – if the main verb is present or future – you’ll see one of two subjunctives. The present subjunctive is used when the action of the subordinate clause is contemporaneous with the action of the main verb; the perfect subjunctive when the action of the subordinate clause takes place prior to the action of the main verb.
If we’re in secondary sequence – if the main verb is in the past – it’s the same deal with the other two subjunctives. This time the imperfect subjunctive is used when the action of the subordinate clause is contemporaneous with the action of the main verb; the pluperfect subjunctive when the action of the subordinate clause takes place prior to the action of the main verb.
You can now begin to see that the subjunctive’s role is in defining the timing of one clause in relation to the other. Note also that, given that many Latin texts are in some way narrations of past events, you tend to see secondary sequence – lots of imperfect and pluperfect subjunctives – more regularly in the average Latin text.