This is a quite difficult issue to understand, Subjunctive entails a problem hard to ignore. Our recommendation is to try to get the general concept and to practice a lot with exercises and specific texts. This practice, together with the study of grammar will help the student to familiarize with Subjunctive.
First of all let us see what is Mood. Using a simple idea we could see Mood as a bag where all tenses can be found. So in the “Indicative bag” we find Present, Preterit, Imperfect Tenses as web as in the “Subjunctive bag” we find another Present, Preterit and Imperfect Tenses, etc.; In the “Indicative bag” time is coral, thus present, past and future are values that differentiate the Tenses and determine their names. In the “Subjunctive bag” mood prevails over time.
Ex. Let's see some examples:
a) Quiere que vengas con nosotros .
He wants you to come with us
b) Le gustaría que vinieras con nosotros.
He would like you to come with us
The main characteristic of Subjunctive Mood is that it does not introduces new information. Subjunctive expresses unreality, hypothesis, desire, doubt, etc, but these elements can be explained with tenses that do not necessarily belong to Subjunctive Mood.
Ex . c) Quería que vinieras, pero al final no te invitamos .
He wanted you to come but at the end we did not invite you.
d) Dijo que venía pero al final cambió de idea.
He said he was coming but at the end he changed his mind.
e) Quiere que te tomes unas vacaciones con él.
He wants you to take some holidays with him.
f) Le gustaría irse de vacaciones contigo.
He would like to go on holidays with you.
Main uses of Subjunctive Mood:
Indicative: Cierto , true, seguro , sure, evidente , evident, etc.
Subjunctive : Bueno , good , importante , important , fácil , easy , lógico , logic , mejor , better , probable , probable, posible , posible, justo , fare , horrible , horrible, normal , normal, etc.; and their opposites . In the Subjunctive Mood we find an assessment (positive or negative) of situations or facts.
In the Subjunctive Mood there are two main Tenses, Present and Imperfect. They can be easily differentiated by examples and specific exercises :
Present Tense:
Used to talk about Present and Future:
I need you to tell me your problem
Maybe he will not come
When you come we will talk
Imperfect Tense:
Used to talk about an action that can be present, past or future:
Si tuviera un hijo, no podría trabajar en esa oficina.
If I had a child I could not work in that office.
Dijo que viniera después de comer y así lo hice.
He told me to come after lunch and so I did.
Si tuviera dinero, haría ese viaje.
If I had money I would do that trip.