1. Modal verbs
of
deduction

2. may / might /could : you think
something is possibly true
can’t : you are sure something is
impossible / not true
must : you are sure something is true

3. Is Sally at home now?

4. We use indicative tenses
to say what we know

5. Sally is at home now.
I know

6. We use modals when we
suppose, deduce, imagine

7. Sally must be at home now.
I suppose

8. The choice of modal verb says
how certain we are

9. Sally
may
might
could
can’t
must
be at home now.

10. Sally
may
might
could
can’t
must
be at home now.
it’s possible

11. Sally
may
might
could
can’t
must
be at home now.
I’m sure she isn’t
(even if I don’t
know)

12. Sally
may
might
could
can’t
must
be at home now.
I’m convinced
(even if I don’t
know)

13. Notice the opposites!!

14. Sally must be at home now.
Sally can’t be at home now.

15. Modal verbs of deduction
have a continuous form

16. Something
may
might
could
can’t
must
be working.

17. Use the continuous infinitive
after the modal

18. continuous infinitive
be working
be speaking
be thinking
etc.

19. But, be careful!!

20. Sometimes the meaning
changes depending on whether
you use the simple or the
continuous infinitive

21. e.g.
He can’t speak French.
He can’t be speaking in French.

22. e.g.
He can’t speak French.
He can’t be speaking in French.
he doesn’t
know how

23. e.g.
He can’t speak French.
He can’t be speaking in French.
I don’t believe
he’s doing it

24. Modal verbs of deduction
have a past form

25. Something
may
might
could
can’t
must
have worked.

26. Use the perfect infinitive
after the modal

27. perfect infinitive
have worked
have spoken
have thought
etc.

28. But, be careful!!

29. Other meanings of the modals
don’t use the perfect infinitive

30. e.g.
He couldn’t speak French.
He can’t have spoken in French.

31. e.g.
He couldn’t speak French.
He can’t have spoken in French.
he didn’t know how
when he was
younger

32. e.g.
He couldn’t speak French.
He can’t have spoken in French.
I don’t believe he
spoke in French

33. Let’s practise

34. Convert continuous modals
to perfect
and perfect modals
to continuous

35. continuous modal perfect modal
She might be calling.
They can’t have studied.
He must be going by bus.
She won’t have cooked dinner.
He will be wearing the jacket.
They won’t have taken their exams.
I must be dreaming it.
They may have argued.
He can’t be choosing.
You must have paid a lot.
She will be enjoying her holiday.

36. continuous modal perfect modal
She might be calling. She might have called.
They can’t be studying. They can’t have studied.
He must be going by bus. He must have gone by bus.
She won’t be cooking dinner. She won’t have cooked dinner.
He will be wearing the jacket. He will have worn the jacket.
They won’t be taking their exams. They won’t have taken their exams.
I must be dreaming it. I must have dreamt it.
They may be arguing. They may have argued.
He can’t be choosing. He can’t have chosen.
You must be paying a lot. You must have paid a lot.
She will be enjoying her holiday. She will have enjoyed her holiday.

37. Let’s put all that into practice
http://learnenglish.britishcouncil.org/es/grammar-reference/modals-deduction-present
http://www.perfect-english-grammar.com/modal-verbs-of-probability-exercise-1.html