What is the difference between the present subjunctive and the imperfect subjunctive?

The difference between these two tenses is simple, it's only a matter of when you use them. Once you have learned what phrases and situations need the subjunctive all you have to remember is that if the phrase/situation is in the present or future tense then you use the present subjunctive. If the phrase/situation is in the preterite, imperfect or conditional tense then you use the imperfect subjunctive.