def past_active(self):
        """
        Weak verbs
        I
        >>> verb = WeakOldNorseVerb()
        >>> verb.set_canonic_forms(["kalla", "kallaði", "kallaðinn"])
        >>> verb.past_active()
        ['kallaða', 'kallaðir', 'kallaði', 'kölluðum', 'kölluðuð', 'kölluðu']

        II
        >>> verb = WeakOldNorseVerb()
        >>> verb.set_canonic_forms(["mæla", "mælti", "mæltr"])
        >>> verb.past_active()
        ['mælta', 'mæltir', 'mælti', 'mæltum', 'mæltuð', 'mæltu']

        III
        >>> verb = WeakOldNorseVerb()
        >>> verb.set_canonic_forms(["telja", "taldi", "talinn"])
        >>> verb.past_active()
        ['talda', 'taldir', 'taldi', 'töldum', 'tölduð', 'töldu']

        IV
        >>> verb = WeakOldNorseVerb()
        >>> verb.set_canonic_forms(["vaka", "vakti", "vakat"])
        >>> verb.past_active()
        ['vakta', 'vaktir', 'vakti', 'vöktum', 'vöktuð', 'vöktu']

        :return:
        """
        forms = []
        stem = self.sfg3et[:-1]
        forms.append(stem+"a")
        forms.append(self.sfg3et+"r")
        forms.append(self.sfg3et)
        forms.append(apply_u_umlaut(stem)+"um")
        forms.append(apply_u_umlaut(stem)+"uð")
        forms.append(apply_u_umlaut(stem)+"u")
        return forms