When do you use the subjunctive?

You can remember when to use the subjunctive using the acroynm WEIRDO:

Wishes, hopes and desires e.g. quiero que, espero que, deseo que 

Emotion e.g. me gusta que, me soprende que, tengo miedo de que

Impersonal expressions e.g. es necesario que, es importante que, es bueno que 

Requests/recommendations e.g. prohibo que, aconsejo que, permito que  

Doubt/denial e.g. dudo que, no creo que, es posible que 

Ojala (the word ojala - I hope, I wish - is used like a verb in the present subjunctive to express wishes and hopes)