The subjunctive mood is indeed a tough issue. There are two main cases when you will use it on the GMAT. The first is after a "bossy verb," such as "require," "request," "implore," etc., basically any verb used to express the desire for someone else to do something. Not all of these verbs require the subjunctive, however. Some require the simple inifinitive in the next verb.
Two examples:
I want you to go to the store. ( to go = infinitive )
I request that you go to the store. ( go = subjunctive present tense )
In the example above, "want" and "request" are the "bossy verbs." Note that "want" goes with the infinitive. "I want that you go" would be awkward, as would "I request you to go." Some bossy verbs can be used in either case; sound it out for each of them, but know that these are the two basic forms.
Now, how is it that we form the subjunctive? In the case above, following the verb "request," we have "go." We form this by taking the infinitive "to go" and simply deleting the "to." This forms the subjunctive in the present tense. Note that we need the word "that" in between the bossy verb and the subjunctive.
Thus, the overall structure will be:
subject + bossy verb + "that" + new subject + subjunctive
The subjunctive is also used to express a sense of unreality. In these cases, we use what is sometimes called the "past tense subjunctive." These forms generally follow words such as "if," "as if," or "as though." We form the past tense subjunctive in the same way as we form the past perfect tense. The only exception is for the verb "to be." This verb always becomes "were" in the past tense.
For example: "If I had known you were coming, I would have baked a cake." "had known" and "were " are both past tense subjunctives. We use these to express that the speaker did NOT know the other person was coming. Note that the conditional "would have baked" is used, because we have an if...then clause.
There are other uses of the subjunctive as well, but the two above are the only ones that you are likely to see. For a more thorough explanation, check our strategy guides, and cross-reference with this:
http://en.wikipedia.org/wiki/Subjunctiv ... in_English