What is the subjunctive?

In Spanish there are two different moods. One of these is called the indicative, which you'll already be familiar with. It is used for making factual statements, or describing obvious qualities in a person or object. 

E.g. A mother is asked where her son Juan lives, she replies:

 Juan vive en Madrid (Juan lives in Madrid)

This is a factual statement, as the mother knows where her son lives.

The other mood is called the subjunctive. This is used to express desires, hopes, conjectures, and possibilities. More importantly, when using the sunjunctive, verbs are conjugated slightly differently. Let's look at the previous example, and see what that concept might look like in the subjunctive. 

E.g. Two people are discussing where their childhood friend Juan might now be living. They are not sure, beacuse they haven't heard from him for a number of years. One of them says:

Es posible que Juan viva en Madrid

By using the subjunctive, they are conjecturing as to where Juan lives, and expressing that they do not know for certain.