I don't understand when I use the imperfect tense and when i should use the perfect tense!

The imperfect and perfect tenses are the two main past tenses used in french but it can be a bit tricky knowing when to use each one!I find that the easiest way of thinking about it is to think of the imperfect tense as a description or an uncompleted action whereas the perfect tense is used for a specific event or completed action.For example:J'étais à l'école quand il a commencé à pleuvoir. Can you tell me which part of the sentence is the imperfect tense and which part is the perfect tense and why?Another time you would use the imperfect tense is when we would say 'I used to do...' in English.For example, 'I used to swim'. would be translated by 'Je nageais.'