Used to indicate doubt, wishful thinking or uncertainty, the past subjunctive is a polite way to express ideas and is called the "past" subjunctive because the verb forms are in past tense.
When we have the money, we will purchase a new television: wenn (when) wir (we) das (the) Geld (money) haben (have), dann (then) kaufen (will purchase) wir (we) einen (a) neuen (new) Fernseher (television).
If we had had the money, we would have bought a new television - Wenn wir damals das Geld (the money) gehabt hätten (had had), hätten wir (we would have) einen neuen Fernseher gekauft (bought).