In what situations do I use the subjunctive?

The use of the subjunctive can be quite daunting in Spanish, as there is no real equivalent in English. In general, it is used in situations of possibility or uncertainty, and is always directly preceeded by 'que'. The following acronym can help you remember its most common uses:

W wish

eg: mi madre quiere que yo limpie mi cuarto

E emotion

eg: me alegre que vengan a la fiesta

I impersonal expressions - Note: only in positions of uncertainty, so with 'es posible que', but not with 'es verdad que'

eg: es necesario que coma bastante

R recomendation/regret

eg: se recomendo que haga ejercicio dos veces cada semana

D doubt

eg: dudo que sea verdad

O ojala que (if only, I hope)

eg: ojala que podamos ir a vacaciones esta fin de semana