speculation and deduction

Uses of modal verbsMíriam Tomàs

Logical assumptions and deductions

Logical assumptions and deductions

▪ Must:▪ is used in affirmative sentences and expresses positive

logical beliefs.▪ To state something about which you are certain that is

true.

must

You have been practising all day, you must be exhausted!

must

Sean and Jennie must be going out again. I’ve seen them talking very lovey-dovey, you know.

Can’t and couldn’t

▪ Can’t and couldn’t are used in negations and express negative logical assumptions. That is to say, you use them when you’re certain that something is not true or real.

Monsters can’t be real!

Can’t and couldn’t

Someone’s been at home, but it couldn’t be James, he was at work.

Possibility

Possibility

+ present infinitive + perfect infinitive

Can CouldCould MightMay / Might would

Possibility

▪ Can + present infinitive expresses general possibility, something theoretically possible.

You can have milk or juice.

Possibility

Could / may / might + present infinitive = It is possible/likely, perhaps…It is used to show that something is possible in a specific situation.

You should keep that number, it may be useful one day.

Possibility

Could / may / might + present infinitive = It is possible/likely, perhaps…

NOTE: we can use can/could/might in questions, but not MAY.

Do you think you can/could/might give me an answer today?

Possibility

▪ Could / might / would + perfect infinitive

It refers to past possibility, something that was possible but didn’t happen.

You might have changed jobs.

They would have been a perfect match, but something got in their way.

Obligation, duty and necessity

Obligation, duty and necessity

Must

Have to

Should / ought to

Need to

Obligation, duty and necessity: must

▪ Must expresses duty, strong obligation to do something, as this something is essential. It also serves to express moral obligation coming from the very speaker.

I must explain what happened.

Obligation, duty and necessity: have to

▪ Have to expresses strong necessity or obligation. We normally use it when somebody other than the speaker has decided that something is necessary.

The landlord said that we have to pay

the rent tomorrow.

Compare…

I must finish the report by 7.00.

You have to finish the report by 7.00!

Must and have to

Do you insist that I…?

▪ Must I stay for the whole meeting?

Is it necessary for me…?

▪ Do I have to buy a new phone to replace the old one?

These two verb constructions have different meaning in questions.

Obligation, duty and necessity: should and ought to

▪ Should and ought to express weak obligation or duty. Also, advice. These are less emphatic than must, have to.

You should think it twice before you make a final

decision.

Obligation, duty and necessity: need

▪ Need expresses necessity (it is necessary to).

I need to tidy up my room today.

Absence of necessity

Needn’t / don’t have to / don’t need to

▪ These structures are used with the present infinitive, meaning that it’s not necessary to do something in the present or future.

You don’t need to drive me home, I’ll take a cab.

I needn’t wake up early today. It’s Saturday!

Didn’t have to / didn’t need to

▪ These two expressions are used when it wasn’t necessary to do something. We don’t know if it was done or not.

He didn’t have to / didn’t need to buy the newspaper. (we don’t know if he bought the newspaper or not.)

Needn’t + bare perfect infinitive

▪ It was not necessary to do something but, even so, it was done.

(You brought flowers even though there’s plenty of them already)

You needn’t have brought flowers!

Prohibition

Mustn’t and can’t

▪ They are used to say that it is forbidden to do something. Either because it is against the laws, rules, or because you are not allowed to do it.

You mustn’t / can’t use a dictionary during the exam.

Criticism

Criticism

▪ Could / should / might / ought to + perfect infinitive are used to criticise someone’s actions or lack of them in the past = it would have been better if you had…

You could have been on time for the meeting.

You ought to have fed the cat and dog.

Now it’s your turn!Go to page 156 to complete exercise 3.

Complete the sentences with the following modal verbs forms and explain their meanings

Must haveShould haveMight haveCould haveCan’t haveShouldn’t haveMight not haveNeedn’t have

▪ That can’t have been John you saw at the match – he’s out of the country at the moment.



▪ You needn’t have brought sandwiches – there’s a perfectly good cafeteria.



▪ What a mess you’ve made! You should have been more careful.



▪ I don’t know why she didn’t ask me how to do it – I could have easily helped her.



She must have thought you were too busy – I can’t think of another reason.



Martin really shouldn’t have borrowed my tennis racket without asking first.



I don’t know why Fiona’s so late; do you think she might have got lost coming here?



She might not have managed to find the time to come. You know how busy she is.

Great job!

speculation and deduction

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