How do I know when to use the subjunctive?

This is a really common concern because, even though we have the subjunctive in English, it isn't used nearly as commonly as in Spanish and is often used incorrectly!

The first thing to remember is that the subjunctive is a MOOD, not a TENSE. This means that rather than referring to when an action takes place, it refers to how the speaker feels about the action. There are two moods in Spanish, the INDICATIVE and the SUBJUNCTIVE. Most of the time, you will use the indicative, which expresses certainty and objectivity. In contrast, the subjunctive is used when there is uncertainty or subjectivity, for example, when the speaker expresses doubt, as in the following example. 

Dudo que ella vaya a aprobar sus examenes. 

The "dudo que" (I doubt that) introduces doubt, and so the next verb, "ir", must take the subjunctive. 

There are several other factors which trigger the subjunctive, such as desire (quiero que hagas..), fear (temo que vaya....), emotion (me alegro de que sea..) and recommendations (sugiero que hagas..). 

It is also important that the subjunctive is always triggered by "que", and the subjunctive clause cannot be introduced without it. 

This is a very brief explanation of a very complicated element of language learning, so please don't worry if it's taking you a while to get your head around it! As you continue to encounter the subjunctive, it will become more and more natural to know when to use it.