How do I determine when to use the subjunctive?

When dealing with the subjunctive, there are two acronyms which, if committed to memory, will help you massively with learning in which situations the subjunctive needs to or needn't be used.

When it is NOT needed:

VOCES

Verdad (truth),

Obvio (obvious)

Cierto (certain)

Evidente (evident)

Seguro (sure)

When it IS needed:

WEIRDO

Wishes

Emotions

Impersonal

Requests/Recommendations

Doubt

Ojalá

These two acronyms will cover the most common of situations in which you may find yourself struggling to decide whether or not to use the subjunctive. I hope this helps!