Earlier in this unit, we studied the option mood, which lets us talk about what should happen. Now we will study a form that lets us talk about what would have happened. The difference between "should" and "would have" is subtle. For "should," we express a wish about the future, which has not yet happened. For "would have," meanwhile, we express a desire to do something in the past to change events and conditions in the future. With that definition in mind, the Sanskrit version of "would have" makes sense; it is a past verb formed with the future stem.
यदि नरा अगमिष्यन् तदा हतो ऽभविष्यम्
yadi narā agamiṣyan tadā hato 'bhaviṣyam
If the men would have come, I would have been killed. (Literal translation)
If the men had come, I would have been killed. (More readable translation)
This form, which is usually called a conditional verb, is rarely used. Grammatically, it is the ordinary past tense as formed with the stem of the ordinary future tense. Thus, the verb uses a future stem to talk about the past.