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

The imperfect tense is used for an action that was ongoing in the past (Je lisais un roman = I was reading a novel). These verbs are often incomplete, hence, imperfect. An example of this is the sentence:Je lisais un roman lorsque tout d'un coup, la téléphone a sonné. (I was reading a novel when all of a sudden, the telephone rang). The reading of the novel was interrupted by the other past action (la téléphone a sonné). This action is a complete action in the past so it takes the perfect form.