What is the subjunctive and why is it so important?

21 years of speaking English and what can I say? I am a boss. I get it right every time. I suppose that’s what you get for being blessed with it as your mother tongue. I haven’t had to memorise our atrocious number of irregular verbs or struggle over pronouncing though (thoe), ought (awt), through (throo), tough (tuff), cough (coff) ...and, well, I could go on. I don’t pretend I’m not fortunate, given that everyone else seems to want to learn English and this “just knowing” is of no use to anyone when I have to explain the rules.