In general could be said that the subjuntivo expresses something not real, something intended, something not-defined, desired or feared, a recommendation or necessity. It means that the things that the speaker talks about mostly happen in his head not in reality. He desires, hopes, wishes, fears, suggests that the things he is talking about would happen.

I want that you come.	correct: Yo quiero que vengas incorrect: Yo quiero que vienes
I demand that you spend less money.	correct: Exijo que gastes menos dinero. incorrect: Exijo que gastas menos dinero
He insists that we do the work.	correct: Insiste en que hagamos el trabajo incorrect: Insiste en que hacemos el trabajo

In English the sentences are put like a fact and a conjunctive would make matters even worse:

I want that you would come.
I demand that you would spend less money.
He insists that we would do the work.

In some cases the conjunctive do work in English. Still in Spanish there is only one possibility.

There is nobody who knows. or There is nobody who would know	correct: No hay nadie que lo sepa. incorrect: Na hay nadie que lo sabe.
There is nobody who has read the book. or There is nobody who would have read the book.	correct: No hay nadie que haya leido el libro. incorrect: No hay nadie que ha leido el libro.