MODOS (modes/ways/forms of seeing/interpreting/dealing with events)

INDICATIVO -----> The "realm" of certainty, reality and facts. It expresses actions, states or facts the speaker considers to be certain.

SUBJUNTIVO -----> The "realm" of uncertainty and subjectivity. It expresses opinions, interpretations, wishes, hopes, desires or the speakers attitude toward events, as well as actions or states that he/she views as uncertain.  

Why must there be both an indicative and a subjunctive mode?

People recognize the need to express ideas in two entirely different realms of thought. On the one hand, there is the objective world of reality. On the other hand, there is the realm of all areas other than those of objective fact.

The indicative mode is used to express that world of reality: what is, what was, what will be.

The subjunctive mode is used to convey ideas in the realm of all areas other than those of objective fact: concepts that are hypothetical, contrary to fact, those which embody the expression of feelings of the speaker toward a state or action.

Although Spanish uses the subjunctive much more and in many more instances than English, the function of the subjunctive in the two languages is not entirely dissimilar. English often does not use the subjunctive to convey ideas expressed by the Spanish subjunctive, but it does have other constructions to express these very same ideas.

Juana trabaja mucho. (Juana works a great deal.) INDICATIVO

Deseo que Juana trabaje mucho. (I wish that Juana would work a great deal.) SUBJUNTIVO

Pedro viene en seguida. (Pedro is coming right away.)IND.

Es preciso que Pedro venga en seguida. (lt is necessary that Pedro come right away) SUBJ.

El chico habla español (The boy speaks Spanish.) IND

Sugiero que el chico hable español. (I suggest that the boy speak Spanish.) SUBJ.

The IND. deals with objective facts that are taking place, will take place, or that took place. The SUBJ. expresses, among other things, hypothetical states or actions, or feelings of the speaker either toward the facts or toward hypothetical states or actions.

Ejemplos de SUBJ. en inglés:

The doctor recommends (that) he take his meds with food.
The law requires (that) you be 18 to vote.
If I were rich I would travel the world.

¡OJO! con los verbos de comunicación:
(decir, telefonear, escribir, etc)

Laura dice que vas a cambiar el día del examen. (She is simply stating a fact, she is just communicating information.)
Laura dice que cambies el día del examen. (She is telling you to change the date. She is trying to influence your behavior.)

¡OJO! con los verbos pedir y preguntar:

Preguntar is used to ask questions, and is NOT followed by the SUBJ.
No te pregunto si tienes la tarea ahora, sólamente si la vas a traer el viernes.

Pedir is used with the SUBJ. to ask somebody to do something.
No te pido que hagas la tarea ahora, sólamente te pido que la hagas para el viernes.

¡OJO! con los verbos dudar y negar: