In this use the subjunctive occurs in a subordinate clause to express purpose. This usual construction employs ἳνα, However occasionally ὃπως or ὣς is used.
When the present subjunctive is used the action of the verb is prolonged or repeated.
When the aorist is used a single action is described or there is no stress on the continuation of the activity.
When the perfect subjunctive is used the completed state of the probable action is emphasized.