3. Main
characteristics
1. They do NOT take –s, -ing, -ed
e.g. He can run very fast. (NOT : He can runs very fast.)
2. They form questions, negations and short answers without the auxiliary
verb “do”.
e.g. May I go now? (NOT: Do I may go now?)
They shouldn’t eat so much. (NOT: They don’t should eat so much.)
3. They are only followed by a bare infinitive except ought which is followed
by a full infinitive.
e.g. You must study harder. (NOT: You must to study harder.)
4. They do not form all tenses. They usually have present/past tense.
e.g. She can speak Japanese. (Present Tense)
She could walk for days. (Past Tense)
4. Must – Have to
(Obligation – Necessity)
1. Must / Have to express obligation and necessity.
e.g. He must be at home before 9.
My mum has to wear a uniform at work.
2. Must can only be used in the present tense.
We use have to to form all other tenses.
e.g. I’ll have to work till late on Thursday.
We had to call the doctor last night.
3. We also use Must to give advice.
e.g. You must talk to your parents about this.
5. Mustn’t – Needn’t
(Prohibition – Lack of Necessity)
1. Must not/ Mustn’t = (You aren’ t allowed to/it is against the rules)
expresses prohibition.
e.g. He mustn’t park here. (It’s against the rules.)
2. Needn’t (It isn’t necessary) expresses lack of necessity.
We can use don’t/doesn’t have to to instead of needn’t with
no difference in meaning.
e.g. You needn’t/don’t have to do all the exercises.
6. Can – Could – Be able to
(Ability)
1. Can expresses ability and can’t expresses lack of
ability in the present.
e.g. Marian can climb a tree but she can’t swim.
2. Could expresses general ability in the past.
e.g. Perry could sing for hours when she was five
but now she can’t.
3. Was/Were able to (managed to do) expresses ability that we had in the
past in a particular situation. It shows that someone did something, but
with effort.
e.g. It was a very tricky question, but Beth was able to answer it.
7. 4. Couldn’t expresses lack of ability in the past, either generally or in a
particular situation.
e.g. Sheryl couldn’t ride a bike when she was two. (Generally)
Paul tried but he couldn’t open the door. (In a particular situation)
We use Could (NOT: was/were able to) with the verbs see, hear, feel,
smell, taste, understand, remember, guess etc.
e.g. Jim could feel that something was wrong the moment he entered
the room.
Can has only present and past tenses. We use Be able to to form all the
other tenses (future simple, present perfect, etc.).
e.g. I hope I’ll be able to pass my driving test.
8. Can – Could
(Permission – Request - Suggestion)
We also use can – could to:
1. Ask for permission. Could is more polite than can.
Can I play with that? Could you pay attention,
please?
9. 2. Make a request. Could is more polite than can.
Dad, can you help me, please? Could you help me, please?
3. Make a suggestion.
We can/could visit the museum tomorrow.
10. Can – Can’t
(Giving / Refusing Permission)
1. We can use can to give permission.
e.g. You can borrow my car.
2. We can use can’t to refuse permission.
e.g. You can’t go out, it’s late.
11. Must – Can’t
(Logical Assumptions)
1. We can use must for positive logical assumptions.
e.g. Jason must be at work. (I’m sure he’s at work.)
2. We can use can’t for negative logical assumptions.
e.g. Sarah can’t know about this. (I’m sure Sarah
doesn’t know about this because no one told her.)
