In what instances would one use the imperfece subjunctive in Spanish?

The imperfect subjunctive can be introduced by four different tenses: the preterite, the imperfect, the conditional and the past perfect. It is often used in clauses like "If I were younger, I would travel a lot" (Si fuera más joven, viajaría mucho). Like the present subjunctive, the imperfect subjunctive can be used to express doubt, referring to unlikely events and possibilities.