Roman languages like French and also Spanish have the specialty of the subjunctive. The subjunctive is one of the most complicated chapter in the Spanish grammar together maybe with the sequence of tenses. The reason for this is that there is nothing alike in the English grammar. Since one of the uses for the subjuntivo are conditional clauses some might think that it is similar to the conditional. Unfortunately the subjuntivo is much more complicated than that.

In English conditional clauses are formed with a combination of past tense and conditional.

If I had money I would buy a car. = Si tuviera dinero, me compraría un coche

The Spanish uses the subjuntivo (in the first half of the sentence, the one with si) for this purpose. In the second half the conditional is used.

First thing we notice is that there is a subjuntivo and a conditional. Which actually means that the subjuntivo cannot be identical with the English conditional (since there is still a Spanish conditional to relate to).

The idea to differentiate between an imagined, desired, feared or hoped situation and the reality is more common than thought, developing a clear set of rules and a clear morphology to form sentences of this kind. Not only Roman languages know the subjunctive but even Persian

Man mitarsam che u beravid = I fear that he would come. (Man mitarsam che u miravad is incorrect)

The Persian "subjuntivo" the eltezami is absolute identical with the subjuntivo of Roman languages, even in small details. The formation is clear and stable and native speakers notice when used incorrectly.

We will give it to him when he has paid us.

Actually you do not know whether he is going to pay or not. Therefore, you cannot be sure that you will ever give it to him. The English version is - if one might say so - not quite exact at two points. First the future says definitely that we will give it to him, second perfect actually has the meaning of something that happened in the (timely close) past that has an influence on the present. When thinking about it the payment did not actually happen. Well, let's not get too confused. Just for you to know. The Spanish have a solution that is exact and can't be misunderstood.

First he insulted me then he talked to me as if nothing had happened.

As if shows that the situation is actually not real. However, the sentence is put into past perfect (which is used to describe actions that have happened before something else happened). Again also for this situation the Spanish have a construction that doesn't give room for doubts.

Primero me insultó y después me habló como si no hubiera ocurrido nada.

I doesn't know anything that would be interesting for us.

This unreal sentence (that would be interesting for us) is put into conditional even though there is no condition related to this sentence. The Spaniard says:

No sabe nada que nos pueda interesar.

There are loads of examples for this kind of assumptions. For instance:

There is nobody who knows. / There is nobody who would know. / There is nobody who could know.
There is nobody who bakes a cake. / There is nobody who would bake a cake. / There is nobody who could bake cake.

No hay nadie que sepa.
No hay nadie que haga un pastel.

Three possibilities for one sentence and if looking at it closely all three are somehow imprecise. And since the Spaniards actively use the subjuntivo and it's not only something in old books we'll now have a look at it a bit more closely.