As a rule of thumb use the subjunctive of the aorist (the "simple", as you call it) when you are not sure what to use. I notice that a common mistake foreign speakers make, is using the present subjunctive instead of the aorist subjunctive, even when they are fluent speakers, and that gives away they aren't natives. For example, it's very seldom that we'd use "να πηγαίνω". In most cases it's "να πάω". Having said that, the present subjunctive is used when you need to emphasize the continuity or the repetitiveness of an action while the aorist subjunctive is used in all other cases. For example: Θέλω να πηγαίνω σινεμά κάθε βράδυ. Θέλω να πάω σινεμά. (απόψε/αύριο/κάποια στιγμή/με την Μαίρη/μόνος κτλ.) Again, it's a rule of thumb but it will get you going in most cases. :) (and here's a nice song about some Mary who'd like to go to the movies... ;) https://www.youtube.com/watch?v=jzAVZYHLLWg )