Modal verbs are used to express ideas like ability, permission, obligation, prohibition, advice, and possibility. They have the same form for all persons and are followed by an infinitive without "to." Modal verbs include can, could, may, must, should, ought to, and had better. They are used to talk about ability, request, obligation, advice, possibility, and deductions or speculations about the past. Modal perfects combine modal verbs with the perfect aspect to discuss past ability, deductions, speculation, hypothetical situations, criticism, and lack of necessity in the past.

1. MODAL VERBS
• have the same form for all persons:
I can, you can, he can…
• do not use do / does to form the negative or
interrogative:
Must you come so late?
I shouldn’t be so lazy, etc.
• are followed by the infinitive without to:
We must go, they should see..

2. ABILITY
Can
Could
Be able to
Present
Past
Future
REQUEST AND
PERMISION
May
Could
Can
Formal
Informal
OBLIGATION
Must Strong obligation
Have to / Need to
Don’t have to /
Don’t need to /
Needn’t
Lack of
obligation
PROHIBITION
Mustn’t
ADVICE
Should
Ought to
Had better
Should have + V3
POSSIBILITY
Must 90%
Can / May
Could / Might
50%
Can’t 0%
Past
Had better always contains
the suggestion of negative
consequences

3. MODAL PERFECTS
POSITIVE DEDUCTION
ABOUT THE PAST
The door is open. Someone must have entered
NEGATIVE DEDUCTION
ABOUT THE PAST
He is on holiday, so he can’t have taken the exam
SPECULATION ABOUT THE
PAST
I haven’t seen Sue for a long time. She may/might have
landed a new job
HYPOTHETICAL ABILITY
ABOUT THE PAST
I’m very clever. I could have become a scientist if I had
studied
CRITICISM OR REGRET Your room is a mess. You should have tidied it up
NO NECESSITY IN THE PAST You needn’t have worried