Hello Rasha!
It took me some time to figure out a rule upon which you can easily distinguish paratatikos from aoristos.... And I didn't really make it to find such a rule...
The only way is the practice of verbs, unfortunately...
As a meaning, paratatikos expresses a sense of continuity for an action which happened in the past, where as aoristos describes a 'perfect', 'done' action which happened in the past.
A proffesional Greek teacher could probably explain it better than me, maybe there's also some grammar rules that i forget. :)