THE SUBJUNCTIVE

The subjunctive forms of verbs could be described as expressing hypothetical actions and situations, as opposed to the real ones expressed by the indicative.
There are three simple subjunctive tenses in Portuguese: present, imperfect and future . Examples:

Pres. subj. Que eu fale, Que eu coma, Que eu decida, etc.
Imperf. subj. Se eu falasse, Se eu comesse, Se eu decidisse, etc.
Future subj. Quando eu falar, Quando eu comer,Quando eu decidir, etc.

The subjunctive mainly occurs in subordinate clauses, i.e. after the conjunction ‘that’ (QUE) and some others, such as "Se" and "Quando" e etc. These are clauses that cannot stand on their own, but depend on a main clause, which is usually in the indicative.
The conjunction ‘that’ is often omitted in English, but "QUE" can never be left out in Portuguese, except in a few very formal turns of phrase.
Ref.: Modern Brazilian Portuguese Grammar - A Practical Guide.