When would I use the subjunctive tense?

In Spanish, the "subjunctive" is described as a "mood" rather than a tense. It can be quite difficult to get your head round but it's just about remembering its different uses; actually forming the tense and remembering the endings is simple. It is used:

-After a wish, piece of advice or a request. For example, after querer que or aconsejar que.

-After an expression of emotion. Such as me gusta que or lo siento que.

-After anything that expresses uncertainty or doubt. E.g Dudo que or Es probable que.

-After a statement that says something should happen, an implication of intention or a description of the conditions of something happening. For example, para que or a pesar de que.

-After cuando or hasta (but only when talking about the future).

-It is also used in some forms of the imperative but you will look at that when studying this form on its own.

There are also some set phrases which you can add into your work if you are having trouble getting your hang around it. Such as Ojala sea... which is translated as "If only it were...". 

Also, you might have noticed that alot of the phrases which "bring on" the subjunctive include "que" - this might help you recognise when the subjunctive needs to be used!