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.
Example
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.
Example
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.