I insist that this food is really good (indicative, because I am simply stating a fact)
I insist that the food be really good (subjunctive, because I am trying to persuade someone to do something)

In Spanish, the subjunctive follows very specific rules that you must learn. For the moment, we are going to concentrate on noun clauses. Noun clauses usually function as the subject of the main clause, the direct object of the main clause, or the object of a preposition within the main clause.

Noun clause as subject:
Es importante que vengas mañana It’s important that you come tomorrow
(If we change the order to Que vengas mañana es importante and compare this sentence to Juan es importante we can see that que vengas mañana is functioning just like Juan, that is, as the subject of es.)

Noun clause as direct object:
Quiero que vengas mañana I want you to come tomorrow
(If we compare this sentence to Quiero patatas, we can see that que vengas mañana is functioning just like patatas, that is, as a direct object.)

Noun clause as object of a preposition:
Tengo miedo de que vengas mañana I am afraid of you coming tomorrow
(If we compare this sentence to Tengo miedo de las arañas, we can see that que vengas mañana is functioning just like las arañas, that is, as the object of the preposition de.)

In order to facilitate the memorization of the verbs and expressions that trigger the use of a subjunctive, it is customary to classify them according to their meaning. Since such a classification is not a matter of science, but only a way to simplify a rather complicated reality, there may be many different ways of grouping these verbs together. One such possible classification is the one offered by the WEDDING mnemonic below. S indicates that the clause functions as a subject, OD as a direct object, and OP as object of a preposition.

W
Want, wish
Yo quiero que Ana venga conmigo (OD)

E

Emotion
Me alegro de tengas buenas notas (OP)
Siento que no puedas venir (OD)
Tengo miedo de que haya una guerra (OP)
Lamento que no tengas dinero (OD)
D
Doubt
Dudo que llueva mañana (OD)
D
Desire, hope
Deseo que te diviertas en México (OD)
Espero que me visites el año próximo (OD)

I

Impersonal

Influence
Es importante que recuerdes mi número de teléfono (S)
Es ridículo que te enfades por eso (S)
Es imposible que no puedas aprender el subjuntivo (S)
Es preferible que te calles (S)

Te aconsejo que no digas nada (OD)
Te digo que te levantes ya (OD)
Te pido que me ayudes (OD)
Te ruego que no hables con ella (OD)
N
Negative opinion
No creo que puedas abrir esa puerta (OD)
No pienso que debas tomar una clase de matemáticas (OD)

G

God willing

“Gustar” type of verbs
¡Ojalá (que) llueva mañana! (Excepción)
Tal vez llueva mañana (Excepción)
Quizás llueva mañana (Excepción)

Me gusta que me llames “cariño” (S)
Me molesta que fumes (S)
No me importa que me insultes (S)

Below are some more verbs that trigger the use of a subjunctive

Verbs of influence or willing:
aconsejar, alentar, decir, dejar, desear, esperar, exigir, insistir en, mandar, ordenar, pedir, permitir, preferir, prohibir, querer, requerir, recomendar, rogar, tener ganas de

Verbs of emotion:
alegrarse de, estar alegre de, estar contento de, estar sorprendido, estar triste de, lamentar, temer, tener miedo de, sentir

Verbs of doubt and negation:
dudar, negar, no creer, no estar cierto/seguro, no pensar

Impersonal expressions:
es + bueno, curioso, difícil, dudoso, estupendo, estúpido, extraño, fácil, horrendo, horrible, importante, imposible, improbable, increíble, inevitable, interesante, irrelevante, malo, maravilloso, mejor, necesario, notable, peor, posible, preciso, preferible, probable, raro, relevante, ridículo, urgente
no es + cierto, seguro, verdad

Verbs like gustar:
agradar, alegrar, dar lástima (miedo, pena, dolor), desagradar, disgustar, doler, encantar, entristecer, fascinar, fastidiar, importar, interesar, molestar, poner furioso (histérico, triste), sorprender

Notes:
1) The conjunction que always has to be present in order to have a subjunctive in a noun clause, and the subjunctive is always after que, not before. The only exceptions are the expressions ojalá, tal vez and quizás.

2) In most cases, there must be two different subjects (that is, one for the main verb and one for the subordinate verb). For example: Quiero que estudies más (I want you to study more – I want; you study). If the subject of both verbs is the same, then an infinitive is preferred: Quiero estudiar más (I want to study more – I want; I study).

3) With many of the Influence verbs, an indirect object pronoun is used before the main verb. This indirect object and the subject of the subordinate verbs are the same person: Te pido que no llores más (I ask you not to cry any more: I ask you; you not cry).

4) You need to use an indicative in the following cases:
a) In the impersonal expressions, when the adjective is obvio, evidente, claro, innegable, indudable, cierto, seguro, or with the noun verdad: Es cierto que no puedo nadar.
b) With the affirmative of creer and pensar: Creo que eres demasiado exigente. In the interrogative, these verbs can take an indicative (usually in Spain) or a subjunctive (usually in some countries in Latin America): ¿Crees que va a llover? ¿Crees que llueva?
c) With decir, when used to express a statement (and not a command): Te digo que no hace frío (I’m telling you it’s not cold).
d) With the verb saber: Sé que en invierno hace frío aquí.
e) With the expression no hay duda de: No hay duda de que hace frío en el invierno.