Everything you've said in Portuguese so far has been about facts — things that happen, happened or will happen. Those verbs are in the indicative: Você fala português. — You speak Portuguese.

The subjunctive (o subjuntivo) is for things that aren't facts yet: what someone wants, hopes or asks for. Espero que você fale com ele. — I hope you talk to him.

It nearly always comes after que, in the second half of a sentence; the first half switches it on. To build it, take the eu form of the present, drop the -o and swap the vowel: -ar verbs take -e, -er and -ir verbs take -a.

Pronoun	Falar (eu falo)	Comer (eu como)	Abrir (eu abro)
que eu	fale	coma	abra
que você / ele / ela	fale	coma	abra
que nós	falemos	comamos	abramos
que vocês / eles / elas	falem	comam	abram

In the subjunctive, eu and você share one form: que eu fale, que você fale. Now try estudar (eu estudo) — the forms you'd use after que.