In the subjunctive mood, present-tense verbs do not change form to indicate the number and person of the subject. Instead, the subjunctive uses the base form of the verb (be, drive, employ) with all subjects.
Also, in the subjunctive mood, there is only one past-tense form of be: were (never was).
Exercises:
Verb tense and mood 1
Verb tense and mood 2
Verb tense and mood 3
Related topic:
Uses of the subjunctive