Lesson 10 – Modal Verbs for Deduction

Today's lesson will focus on using modal verbs for

certainty, probability, and deduction.

"Deduction" means using the information available to make

a guess or draw a conclusion about the facts.

Depending on the information available, you might be more

certain that your conclusion is true, or less certain that your
conclusion is true - and we use different modal verbs to
indicate the degree of certainty.

How certain are you? Use these modal verbs:

100% - Completely or almost certain must, can’t, couldn’t

80% - Expecting to be certain should

50% - Maybe certain might, may, could

Let's look at some examples.

Certainty
Certainty about the Present

When making deductions about the present, we use must if we are sure
something is true and can't if we are sure that something is impossible.

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For example, if you see this guy, you can say:

He must be a chef.
(we are very certain, because of his uniform and
what he is doing)
He can’t be a policeman.
(we are very certain he is NOT a
policeman)
Carla works every day from 9 AM to 5 PM. Right now
it’s 10:30 AM, so…

She must be at work.

She can't be at home.

Put it into practice!

You'll see five pictures accompanied by sentences. For each one, fill in the
blank with must (if you think it is true) or can't (if you think it's not possible).

He _____________ be a vegetarian.

They ______________ be in love.

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 You studied all night. You ___________ be exhausted.

That __________ be right…

She ____________ love her cat.

Certainty about the Past

When we consider some present evidence and draw a reasonably certain

conclusion about what happened in the past, we use must have and couldn't
have plus the past participle of the verb.

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 must have + past participle
when we draw the conclusion that something DID happen

Sheila got a tan. She must have spent a lot of

time in the sun lately.
There was one banana left, but now it's gone.
My husband must have eaten it.

must not have + past participle

when we draw the conclusion that something did NOT happen

The car is still dirty. Paul must not have washed it yet.
He barely touched his lunch. He must not have been hungry.

couldn't have + past participle

when we are certain that something was IMPOSSIBLE

Martha couldn't have taken your notebook; she wasn't even in class
yesterday.
The cookies are gone. But Eric couldn't have reached the cookies
on the top shelf; he must have asked his older brother to get them.

Can’t have is also possible, but it is much less common than couldn’t have.

Here’s an example that illustrates the difference between must not have
and couldn’t have:

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 I don’t see the report here – she must not have printed it out.
(we draw the conclusion that she did not do it)
The printer’s been broken for the past week, so she couldn’t have printed
out the report.
(we know it was IMPOSSIBLE for her to do it)

Put it into practice!

You'll see five pictures accompanied by sentences. For each one, fill in the blank
with must have (if you think it happened), must not have (if you think it
didn’t happen) or couldn't have (if you think it's impossible).

The thief ______________________ gotten in through the window.

I _________________________ left my phone at work; I made a call on the drive home.

She __________________ been happy when she heard the good news.

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They _____________________ bought a house without a loan because they had no
savings.

He looks upset. He _________________________ liked whatever she just said.

“How was your presentation?”

“Great! It _______________________ been better!”

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Certainty about the Future

Making deductions about the future is the same as making predictions. As you
learned in the lesson about future tenses, we can use will or going to for
saying what we believe will happen in the future. When you are quite sure
that your prediction is correct, you can also add the word definitely to
emphasize your certainty:

She's definitely going to love this book - it's by her favorite author.
The kids will definitely be thrilled when we tell them we're going to
Disney World.

Two slightly more formal expressions that can be used for talking about the
future with certainty is saying that something is certain to happen, or is
sure to happen:

Engineers are certain to develop even faster computers.

The country is sure to come to the aid of its ally.

Expectation

Present/Future Expectation

When you expect something to happen (although you are not completely,
100% certain), you can use should/shouldn't for the present or future:

Present:
I took my car to the mechanic yesterday and he said the problem would
be fixed in a day - so my car should be ready by now.

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 Future:
Can you please type up these notes?
It should only take about half an
hour.

Present:
The weather is clear, so our flight shouldn't
be delayed.

Future:
I've written out all the instructions for this task step by step,
so you shouldn't run into any problems when you try to do it.

Put it into practice!

What is one thing you expect to do or one thing you expect to happen within
the next week? Create your own sentence using should:

Ex) Within the next week, I should finish the book I'm reading.

Past Expectation

For expectations about what was supposed to happen in the past, you
can use should have and shouldn't have plus the past participle:

I sent the package three weeks ago with express

mail. They should have received it already.
= I expect that they have already received it

This car is brand new. It shouldn't have broken down.

= I expected it NOT to break down

Because should/shouldn't are also used for giving advice and recommendations,
we can also use should have and shouldn't have for evaluating things in the
past and declaring them to have been right or wrong, good or bad:

Lindsay saw a woman who needed help, but did

nothing. "Lindsay, you should have helped her."

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 I said something mean to my best friend during an
argument. "I shouldn't have said that. It really hurt her
feelings."

Put it into practice!

Think of one thing you regret doing, and one thing you regret NOT doing.
Make sentences about them using should/shouldn't have + the past
participle:

I should have...
I shouldn't have...

Possibility

Present/Future Possibility

We can use may, might and could to talk about things that are possible in
the present and future.

Present:

Where's Fred? He's not in his office.

He may be in the bathroom... or he might be in
the conference room.
Don't eat that mushroom. It could be poisonous.

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Future:

The weather forecast says it may rain tomorrow.

Your daughter is really smart. She could be very successful someday.
We might take a road trip this weekend.

Some people say that might is less certain than may, but in spoken English there
is really no effective difference. It’s probably best to use might. The word may is
less common, and we can only use could in the positive form, not the negative
form, for talking about possibility:

Are you sure that's a good idea?

The boss may not / might not like it when he finds
out. The boss could not like it when he finds out.
I may not /might not be the smartest person in the class, but I
definitely work the hardest.
I could not be the smartest person in the class, but I definitely work
the hardest.

Past Possibility

When talking about past possibilities, we can use... might have / may have /
could have + past participle (for positive possibilities) and might not have /
may not have + past participle (for negative possibilities).

Positive Past Possibilities:

She's not home. She might have gone

to the store.
He may have misunderstood you when
you talked to him yesterday.
The person who stole the
documents could have been one of
the employees.

Could have is usually used in unreal conditions -

when we are imagining a possibility if something in the past had been different:
"If we had started this project earlier, we could have finished on time."
Negative Past Possibilities:

John’s not here. He might not have known about the meeting.
If she hasn’t called you back, she may not have listened to your voicemail
yet.

Remember that couldn't have is only used when we are certain that
something is logically impossible in the past:

She couldn't have taken the car; she doesn't have a key.

Summary
Use must (present) and must have (past) when you are very certain
that something is/was true
Use can't (present) and couldn't have (past) when you are very certain that
something is/was impossible
Use should/shouldn't (present) and should have / shouldn't have (past)
to talk about things you expect to be true, although you don't have complete
certainty
You can also use should have / shouldn't have for judging actions in
the past to be good or bad
Use might (most common), may, or could to talk about present and
future possibilities
Use might have, may have, or could have to talk about past possibilities
For a possibility that something did NOT happen, use might
not have and may not have
Quiz – Lesson 10: Modal Verbs for Deduction
Exercise 1 – Complete the blanks withmust, can’t,ormight:
1. Look at that guy's enormous muscles. He________________ work out a lot.
2. Michelle________________ want to participate in the festival - it seems like the
type of thing she'd be interested in. Why don’t you ask her?
3. She goes camping every weekend. She________________ really love the outdoors.
4. He worked hard on his report, then accidentally deleted the file from his
computer. He________________ be upset.

5. You________________ be right - but I'm going to check to make sure.

6. We're not sure if this painting is an original. It________________ be worth
thousands of dollars.

7. I________________ not be able to go to the football game. It depends on whether

I can get the afternoon off from work.

8. He's working full-time and studying for his Ph.D. That________________ be easy.
9. You just ate a huge dinner! You________________ be hungry again
already! 10.Wow - look at that diamond necklace. It________________cost a
fortune.
Exercise 2 – Now complete the blanks withmust, should, shouldn’t,orcouldn’t:

1. I'm so thankful for your help with this project. I___________________have done it
without you!

2. The repairs I made___________________have fixed the problem, but they didn't -

so I'll have to take another look.

3. It___________________have rained a lot last night - there are puddles

everywhere.

4. I___________________have watched that horror movie; it gave me nightmares.

5. We___________________have known about this; nobody told us anything about it.
6. If you’ve been trying to lose weight, then you really___________________have
eaten all that ice cream.
7. He___________________have finished a 500-page book in a single day!
8. She never showed up. She___________________have forgotten about our
appointment.

9. He broke two of the plates while washing the dishes. He___________________

have been more careful.

10.They ate every bite of their dinner - they___________________have enjoyed the

food.