How and when do I use the imperfect subjunctive?

The imperfect subjunctive is formed by taking the root of the first person preterite (e.g. the verb 'comer' - 'to eat' - becomes 'comi') and adding the following endings:
-iera (e.g. comiera)-ieras -iera-iéramos-iéris-iéran
We use this form when talking about something that might possibly have happened in in the past, or may still happen - so it is generally preceded by the preposition 'si' ('if') and followed by a main verb in the simple conditional form. For example:
'Si fuera presidente, me dedicaría a formar una sociedad más igual' - 'If I was president, I would focus on creating a more equal society'