You class can learn all about how to form the imperfect with -ar and -er/-ir verbs. The slides do cover the only verbs that are irregular in the tense and the situations for which you use the imperfect (plus examples). Go over the included expressions that are commonly used with the imperfect and close with the practice fill-in-the-blank and translation sentences. Since this presentation includes a significant amount of information, make sure your class takes notes while viewing.