How do you know when to use the subjunctive tense?

The subjunctive tense is non existent in English and therefore can be a bit confusing to grasp at first. With practice of course one gets used to it. It is mainly used to describe events that are either abstract or uncertain in their even occurring, such as birthdays in the future. It is also used to convey senses of desire, for example when you want someone else to do something; and for advice, for example “it is important that you go to school”.