The subjunctive mood appears most frequently in dependent (subordinate) clauses.1 A dependent clause may function as a noun (subject or object of verb, object of preposition), as an adjective (modifying a noun), or as an adverb (modifying a verb). Usually the subjunctive will appear when the subordinate clause

1. depends on an idea of command or causation in the main clause
2. reflects an emotion in the main clause
3. depends on an idea of unreality (indefiniteness, doubt, contrary-to-fact) in the main clause.

Noun Clauses
In general, subjunctive is used only where the subject of the main clause and the subject of the subordinate clause are DIFFERENT, in the following situations:

1. Verb of main clause causes or leads to action in the subordinate clause (idea of wishing, asking, desiring, requesting)

2. Verb of main clause conveys feeling or emotion.

3. Verb of main clause signifies doubt or denial.

4. Impersonal expressions.

Observe that it is the meaning conveyed by the verb, not the particular verb used, that determines whether a subjunctive will appear in the subordinate clause. Escribir normally is not a verb of causing, but in 1.c. it does cause the action in the subordinate clause, and a subjunctive has to be used. Likewise creer in 3.c. is used with a meaning of doubt and the subjunctive is used. BUT:
a. No dudo que vendrá.
b. No niego que es buen muchacho.
(Verbs may be of doubt, but are here used for idea of certainty)
(Clause: A secondary phrase, almost always introduced by que.

Adjectival Clauses
The subjunctive will be used if the antecedent (the noun which the clause in question modifies) is either non-existent or indefinite.
1. No hay un hombre que pueda ayudarme.
2. No tengo un libro que pueda prestarte.
3. Quiero ver un hombre que hable español. (Any man who happens to speak Spanish will do: no definite antecedent.)

BUT: Tengo un hijo que habla español. (Definite person)
Existe un hombre que puede ayudarme. (There is a particular man here.)

Adverbial Clauses
1. If the adverbial clause is part of the ESCAPA acronym (en caso de que, sin que, con tal que, a menos de que, para que, and antes de que), the sentence will always require the subjunctive.
a. The next step is to check the tense of your verb. If the verb is in the present or in the future, the adverbial clause will require the present subjunctive.
e.g. Voy con tal que tú vayas.
Iré con tal que tú vayas.
b. If the verb is in the preterite, the imperfect, or the conditional, the adverbial clause will require the imperfect subjunctive.
Fui para que tú fueras.
Iba para que tú fueras.
Iría para que tú fueras.

2. When the action in the subordinate clause is not an accomplished fact (usually indicated by the use of the future tense or a hypothetical situation), the sentence will always require the present or the present perfect subjunctive.
Me iré cuando ellos lleguen.
Aunque vayas, no iré.
Aunque hayas ido, no iré.
However, if the action is completed, the preterite is used.
No me fui hasta que llegaron.
Aunque fuiste, yo no fui.
In repeated or habitual (therefore, completed) actions, either the present or the imperfect is used.
Me voy (habitual) cuando ellos llegan. vs Me voy (planning to leave) cuando ellos lleguen.
Me iba cuando ellos llegaban

The imperfect subjunctive is used with the conditional.
Aunque fueras, yo no iría.
Yo me iría, cuando ellos llegaran.
The pluperfect is used with conditional perfect tenses:
Habría ido aunque tú no hubieras querido.

Subjunctive in Conditions:
The subjunctive (imperfect or past perfect only) is use in the if-clause of a conditional sentence that expresses uncertainty or is contrary to fact; such sentences will have a result clause in the conditional or conditional perfect in English, so they can be easily recognized when you are translating.
a. If I were king, I would banish you. (contrary to fact) - Si fuera rey, te desterraría.
b. If you came, you would be welcome. (hypothetical) - Si vinieras serías bienvenida.
1. If the result clause is in the conditional in English, us the imperfect subjunctive in the if-clause in Spanish and the simple conditional in the result clause.
a. Si fuera rey, lo mandaría al exilio.

2. If the result clause is in the conditional perfect in English, use the past perfect subjunctive in the if-clause and the conditional perfect in the result clause in Spanish.
(If I had been king, I would have killed him.)
Si hubiera sido rey, lo habría matado.

Subjunctive as a Command Form In Spanish:
T the present subjunctive is used for all direct commands except direct affirmative commands in the familiar (singular and plural) - these will be in the imperative.
Subjunctive commands:
Venga Ud. Aquí. (polite, affirmative)
Vengan Uds. Aquí. (polite, affirmative)
No venga Ud. Aquí. (polite, negative)
No vengan uds. Aquí. (polite, negative)
Comamos temprano. (1st person plural command)
No comamos.

Juan, no vengas acá (familiar, negative Juan, ven acá (familiar, affirmative)
Niños, no vengáis acá. Niños, venid acá.

Personal object pronouns will be attached to the end of a command form (subjunctive or imperative) in affirmative commands only; in negative ones, they will come before the verb form:
1. Démelo Ud. (affirmative, attached) - Juan, dámelo.
2. No me lo dé Ud. (negative; before verb) - Juan, no me lo des.

Examples:

1. a. Quiero que venga.
b. Espero que haya venido.

2. a. Quise que viniera.
b. Esperé que hubiera venido.