Subjunctive Conjugation Rules

In the simplest terms, the Subjunctive Mood of the verb is required if the Independent Clause expresses one of the following moods or syntactic relationships to the Dependent Clause:  (I recommend learning the subjunctive as pieces divided into the following categories, and learn them in the sequence that follows)

1. Will, Indirect/Relayed Command, Advice, Prohibition:  Quiero que hables.  Dile que no me llame otra vez.  Te aconsejo que no lo hagas.   Te prohibo que salgas con él.
2. Impersonal Noun Clause   Es importante que lo sepas.   Es bueno que me ames. Es triste que... (all such structures--Es adjective que.... except for Es cierto que..., Es verdad que.., Es obvio que..., es evidente que...--because these concepts are contrary to the non-experienced quality of the Subjunctive Mood) 
3. Doubt, negation, denial, non-existence:  No hay nadie que te pueda ayudar.  No creo que ellos vengan.
4. Description of non-specific thing, Need or desire for a non-specific thing, (whichever, whatever may, might...)    Necesito un apartamento que tenga dos baños.   ¿Hay alguien que hable chino aquí?
5. Emotional Reaction: Me alegro de que...
6. Idiomatic phrases: O sea... (in other words),   sea hombre o mujer (whether it be a man or woman), Tal vez tenga...  (perhaps he has..)
7. On a more sophisticated level, there are also a series of adverbial conjunctions (click here) that also require the subjunctive, but for the sake of this brief, simple explanation, I am leaving most of them out.  The following ones are very hard to avoid, so I will show them to you:

para que  (so that)   is always followed by the subjunctive

antes de que (before) is always followed by the subjunctive

después de que (after) is followed by the subjunctive if the events or conditions described are in the future, or haven't happened yet.

There are many others, but for the sake of simplicity, they aren't mentioned here.  We will deal with them in the 2100 class.  Click here for further instructions if you want them.

COMMON ERRORS TO WATCH FOR

Decir in the independent clause calls for subjunctive only if it is relaying a command.

Deceptive structures that seem to suggest doubt or uncertainty

SI  (if) is NEVER followed by the present subjunctive.   IF is followed by the imperfect subjunctive when the other clause is in the conditional form.  In this case, what is said is understood to be contrary to fact, and the speaker is referring hypothetically to what would happen if reality were other than it is.  

Other deceptive structures

Non-Switching Subject Error for most subjunctive structures, there must be a different subject in each clause.  Failing to follow this is one of the most common errors I see.

I hope that I can go.   Must be changed to "I hope to be able to go."

Espero poder ir.    It is incorrect to say: Espero que yo pueda ir.

Finally, here's a word of advice: The subjunctive mood is not difficult to learn or use if you have the conjugations down.  The rules are many, and the size of the rules can be daunting, but the rules are very elegant and consistent.  Most errors you will make involve forgetting the rules.  In contrast to the problem of the preterite vs imperfect, the subjunctive vs indicative problem is quite clear and rigorous.  Once you get the hang of it, I think you will agree that the subjunctive is rather fun to use. 

The best way to learn it is to FIRST MASTER THE CONJUGATION, then collect a lot of independent clauses and memorize whether or not they require the subjunctive.  Just as with the imperfect vs preterite issue, students tend to focus too much attention on the rules and not enough on the conjugations. 

Some good news about the subjunctive vs indicative issue:

• the rules for when to use subjunctive are finite and pretty logical.  In my opinion, they are much more clear and systematic than the preterite vs imperfect rules. 
• making mistakes generally doesn't cause a large amount of confusion to the listener or reader, though it does sound bad. 
• the conjugations that you need are pretty easy to master.  Once you have assimilated the present tense and preterite tense forms, you can quickly produce all the conjugations of both the present and imperfect subjunctive moods.  One word of warning--the present tense and the preterite tense are the two most difficult tenses to conjugate. 
• using the subjunctive is actually rather fun and opens up lots of new structural and conceptual possibilities for you.