Thank you for using the Jiskha Homework Help Forum. I like to call the Subjunctive the "thinking person's tense" because you need to be aware of certain things! Now you will be making decisions between the Indicative, Infinitive and/or the Subjunctive. If you are looking for a QUICK answer, there is none!
1. First you will learn how to form it. There are one-root verbs and two-root verbs. One-root verbs use the 3rd person plural stem and two-root verbs use the 1st & 2nd person plural stem for the lst & 2nd personl plural forms. There are regular and irregular (avoir/être). There is a Present and a Past Subjunctive, where you'll need to know the verbs that use the auxiliary être and those that use avoir. Look for 2 different subjects usually.
2. There are circumstances when the Subjunctive is mandatory, or optional or even forbidden. Usually if will be in the dependent clause after que (qu') because it is dependent upon the verb or expression in the independent clause. For example: I WANT (verb of wishing) you TO GO. The "to go" is not the Infinitive but the Subjunctive because I do not have a crystal ball and therefore I can not know for a certainty that you WILL go! What is deceptive is that in English it might sound like an infinitive (as in this case), the Present, Future, etc.
3. The Subjunctive relates to the MOOD of a verb. You will be looking at the main verb to classify it. If it is a verb of order, command, desire, wish or emotion, feeling, or opinion or perception you will require the Subjunctive. It conveys the attitude of the speaker. (These are the mandatory uses.)
4. It is optional after superlatives, in certain relative clauses, with certain verbs and adjectives in the interrogative. The affirmative uses the Indicative while negative uses the Subjunctive and Interrogative depends upon the attitude of the speaker!
5. It is forbidden or NEVER used with certainity, fact unless those statements are negative or interrogative. An easy rule is "no emotion, will or restriction = no Subjunctive."
Now, aren't you glad you asked?!! Take it one step at a time and step-by-step you will understand it. You'll have certain adverbs of time to learn as well!