1. MODALS - INTRODUCTION
MODAL VERBS can, could, may, might, must, ought to, shall, should, will and would:
are always used before another verb in its infinitive form without to: He can swim.
never change – they do not add –s, or –ed or –ing.
are immediately followed by not in the negative:
You should not (shouldn’t) be late.
go immediately before the subject in a question:
Could you wake me up?

2. OBLIGATION AND NECESSITY
PRESENT: must and have to
I must/have to go now or I’ll miss the bus.
We only use must in the present tense. In all other tenses we use have to:
I had to work every day last week. (past simple)
I will have to work longer hours. (future)
When something is unnecessary we use don’t have to, or needn’t (NOT mustn’t):
You don’t have to / needn’t take your shoes off inside, but you can if you want to.
Mustn’t is a prohibition (when it’s wrong to do it, it isn’t allowed): You mustn’t smoke inside.

3. ADVICE You should / ought to start looking for a job.
Should and ought to mean the same but we use should more than ought to, especially in negative forms and questions:
They shouldn’t let their children watch that.
What should I do?
When we talk about the past , we use
should + have + past participle
I should have told the truth (= it was a good idea but I didn’t)

4. ABILITY How many languages can you speak? CAN - present ability
He could read when he was three.
She was able to finish on time.
She managed to finish on time
You will be able to operate the machinery at the end of this course.
CAN - present ability
COULD - past ability in general
WAS/WERE ABLE TO or
MANGED TO – specific occasion in the past (not COULD)
WILL BE ABLE TO – future ability

5. DEDUCTION: CERTAINTY AND POSSIBILITY
I haven’t seen the neighbours all week. They must be away.
The man in the photo can’t be Peter because he never wears a hat.
The letter might be / may be / could be from Tony.
She might come and see you tomorrow.
Ed may not know how to find our house.
MUST – certainty
CAN’T – impossibility
MIGHT / MAY / COULD – present and future possibility
we use MAY NOT and MIGHT NOT in negative sentences, not COULD NOT

6. TALKING ABOUT THE PAST
I can’t find my keys. I must have left them at home.
He can’t/couldn’t have got there yet because it will take at least two weeks.
I’m not sure what sort of bird it was that I saw, but it may/might/could have been an eagle.
MUST / CAN’T / COULDN’T HAVE + PAST PARTICIPLE – when we are certain about the situations in the past
MAY / MIGHT / COULD HAVE + PAST PARTICIPLE – possibilities in the past