Here's an ESL lesson to review past modal to express degrees of certainty in the past.
This grammar guide aims to highlight the meaning behind these modal verbs as it is the most problematic area in them. It's a real hands-on learning lesson. This lesson will help you assess your student's knowledge of past modals and also can be used as an introduction to past modal verbs. They include all forms with many examples. You can accompany this worksheet with these exercises to consolidate the subject. The answer key is also included.