Sequence of Tenses
The tense of the subjunctive in a subordinate clause is governed by the rule of sequence of tenses.
If the main verb expresses present or future time, then the subjunctive is in the present or perfect: primary sequence.
If the main verb expresses past time (imperfect, pluperfect and usually perfect) then the imperfect or pluperfect subjunctive is used: secondary or historical sequence.
How to know which tense of the subjunctive to use.
In primary sequence, use the present subjunctive for an action going on at the same time as that of the main verb. Nescio quid dicas. "I do not know what you are saying."
In primary sequence, use the perfect subjunctive for an action that is completed at the time of the main verb. Nescio quid dixeris. "I do not know what you said/have said."
In secondary sequence, use the imperfect subjunctive for an action going on at the same time as the action of the main verb. Nesciebam quid diceres. "I was ignorant of what you were saying."
In secondary sequence, use the pluperfect subjunctive for an action that is completed at the time of the action of the main verb. Nesciebam quid dixisses. "I was ignorant of what you had said."
Although the perfect indicative is usually a secondary tense, sometimes primary sequence is used with it, especially in result clauses. The tense of the subjunctive to be used with a perfect indicative depends on the sense in which the perfect is used (as a simple past [secondary] or as a true perfect [primary], i.e. when translated by "have _____")
A subjunctive dependent directly on a perfect infinitive or a dependent perfect subjunctive requires secondary sequence, even if the main verb is primary.
A subjunctive dependent on an imperfect or pluperfect subjunctive uses secondary sequence.
In a purpose clause, the action of the verb cannot take place before that of the main verb and so only the present and imperfect subjunctives are used in purpose clauses.
To express future time in subjunctive clauses: in primary sequence, use the present subjunctive; in secondary sequence, use the imperfect subjunctive. For greater definiteness, the periphrastic forms -urus sim, -urus essem (the future active participle with the subjunctive of the verb sum) may be used. Temporal particles (such as mox, brevi, statim may be used with the present or imperfect subjunctive to make futurity more clear.