It's an introduction to modal verbs - first some basic rules, then the rules for forming past (not all modal verbs have separate forms for past, some use perfect infinitive - in my experience it is difficult for students to distinguish between the two ...
It's an introduction to modal verbs - first some basic rules, then the rules for forming past (not all modal verbs have separate forms for past, some use perfect infinitive - in my experience it is difficult for students to distinguish between the two ...