How do I know when the subjunctive is needed in Spanish?

The Subjunctive is one of the hardest ideas in Spanish, simply because it is not commonly used in English!  The easiest way I find to remember it is to think of WEIRDO!

Wishes- eg when using deseo que or quiero que! EG: deseo que mi madre me compre nuevos zapatos 

Emotions- when you are expressing you're happy/sad/upset on account of something. EG: me alegro de que estés aquí

Impersonal expressions- these are the nice essay stock phrases to get your subjunctive in easily.  EG: es importante que puedas ver las noticias, or es posible que no vaya.

Recommendations- this is on the suggestion of someone else. EG: mi madre sugiere que vayamos a la playa, or mi colegio recomienda que no durmamos menos que ocho horas cada noche 

Doubt- this is when you're not sure of something or there is a certain level of uncertainty- EG: dudo que vaya or no estoy cierta si entiende 

Ojalá- this is the easy one! just remember that after ojalá (god willing), you have to use the subjunctive!- EG: ojalá que haga sol, or ojalá que esté aquí. 

REMEMBER: if you're unsure, the subjunctive is used in terms of UNCERTAINTY.  If you are stating a definite fact, then the subjunctive is probably not needed!