You have learned the INDICATIVE mood since your first verb conjugation in Spanish. If we look at the present indicative tense, we have certain verb endings for -AR, -ER, and -IR verbs. There are also irregular verbs and stem-changing verbs.
Present subjunctive does pretty much the same thing, including all those irregularities, but the endings use the opposite vowel as you'd expect.
Here's what I mean using two simple, regular verbs: hablar and aprender.
PRESENT INDICATIVE = Yo hablo, Tú hablas, Él habla, Nosotros hablamos, Ellos hablan
PRESENT SUBJUNCTIVE = que yo hable, que tú hables, que él hable, que nosotros hablemos, que ellos hablen
Notice our -AR verb is taking endings that use an E. This is "opposite" to what we're used to!
PRESENT INDICATIVE = Yo aprendo, tú aprendes, él aprende, nosotros aprendemos, ellos aprenden
PRESENT SUBJUNCTIVE = que yo aprenda, que tú aprendas, que él aprenda, que nosotros aprendamos, que ellos aprendan
Now, our -ER verb is taking endings that use an A. The -IR verbs take these same endings.
1. Make the YO form of the present indicate: ESTUDIO
2. Take off the -O: ESTUDI-
3. Add the "opposite" ending: que tú ESTUDIES
Any irregularities that happen in the YO form of present tense will carry through to the present subjunctive. This would mean any stem-changes or changes in the yo form.
PENSAR (e-ie): que yo piense, que tú pienses, que él piense, que nosotros pensemos, que ellos piensen
CONOCER: que yo conozca, que tú conozcas, que él conozca, que nosotros conozcamos, que ellos conozcan
You may be asking, why did I write "que" before all those subjunctive conjugations? This is because subjunctive does not stand alone in a sentence (with a few exceptions). A subjunctive verb will usually have the word que before it to separate the main and subordinate clause in the sentence. Next week we will delve into the larger structure of sentences which use subjunctive and why they do what they do, but for now, I just want to review the actual conjugation of forms.