In Portuguese there are several ways to talk about the future, each with varying shades of meaning. The present subjunctive is used to show that some idea or action is hypothetical. For example,
I hope that you have a pair of sunglasses is hypothetical. So is, Do you want me to speak to her?
HYPOTHETICAL situations include those that express desire, emotion, improbability, give commands, and to express opinion.
espero que; caso que; é importante que; é possivel que; é necessário que; are triggers for this tense.
There is a pattern to the conjugation and rather than explain it, just see if you can get it by example:
-ar present subjunctive regular verbs
|that I speak||que eu fal||e|
|that he speaks||que ele fal||e|
|that we speak||que nós fal||emos|
|that they speak||que eles fal||em|
-er and -ir present subjunctive regular verbs
|that I eat||que eu com||a|
|that he eats||que ele com||a|
|that we eat||que nós com||amos|
|that they eat||que eles com||am|