How do you use the subjunctive mood?

The subjunctive tense in Spanish can't be thought of like the other tenses (present continuous, perfect etc). It is used to reflect the mood of the speaker. In English, the subjunctive is not that common eg.

"If I were a dog"

= "Si yo fuera un perro" (adding "yo" to reinforce "I" as "fuera" is used for both first person and third person conjugations)

It refers to something hypothetical, something unsure, or something impossible. The subjunctive is often follows "que" in a sentence involving the mood. The easiest way to get to grips with the subjunctive is by learning the phrases which introduce it eg

"Espero que ...."

"Quiero que ...."

"Deseo que ...."

"Es imposible que ..."

"Lo que ..."

These are examples of when there is doubt in the statement, so we must use the subjunctive. The subjunctive is also used to express desires and statements which you are unsure about.