How do you know when to use the imperfect tense and when to use the perfect tense?

Simply, the imperfect tense is used to convey an action in the past that was taking place over a period of time e.g. 'Je regardais la télé' = 'I was watching TV' - the act of watching TV was not instantaneous. On the other hand, the perfect tense is used to convey a finished action in the past e.g. 'J'ai vu Paul' = 'I saw Paul' - the act of catching sight of Paul IS instantaneous and completed immediately.