Sovannaphumi School, TTP
Level 8B
Teacher: Bun Sreyphos
Modal Verbs of Possibility and Impossibility
Can’t, Could, may, might must
Modals of Possibility and Impossibility ( Present/ Future)
we use modal verbs Can, Could, May, Might, Must in a sentence when we want to make logical
guess about situations or things happening or to say whether things are possible.
Form: S+ Modal verb+ bare infinitive (Must/ Can/ Could/ May/ Might/)
The choice of using modal verbs depends on the level of certainty (how sure we are about the situation
or things)
Must:

It’s used in affirmative sentences and expresses positive logical assumptions. We normally use
must in a sentence when we make a guess on something based on practical evidence and logical
reasons.
Must  I’m sure/certain that something is possible to happen
Example:
-
It is Monday. He must be at work. (I’m sure he is at work. We know it’s most logical that people
work on Monday.
This must be her house. I can see her car in the garage.
He must live near here because he always walks to work.
Come inside and get warm. You must be freezing out there!
Can’t

We don’t use Must not to expressive the impossibility of situation, instead we use Can’t and
Couldn’t are used in negations and expresses negative logical assumptions
Example:
-
It is Sunday. He can’t/ couldn’t be at work. (I’m sure he isn’t at work. We know it’s most logical
that people don’t work on Sunday.
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Sovannaphumi School, TTP
Level 8B
Teacher: Bun Sreyphos
May, Might, Could, Can

When we want to make a guess on something without any practical solution rather on your
opinion or when you are not sure about the situation or things happening, we use May, Might,
Could, Can
Can is used in affirmative sentences to show that something is possible theoretically or in general, that is
when we are not referring to a specific situation.
Example: This road can be very busy. (In general, not in a specific situation.)

Meanwhile Could, May, Might are used to show that something is likely to happen in a specific
situation.
Example: The roads could/might/may get very busy tomorrow afternoon because there is a
demonstration.
- She's not here yet. She might be stuck in traffic. (I’m not sure what took her so long but I think
traffic jam is the reason why she’s not here)
- He's not answering. He could be in class. (I don’t have any evidence that he’s in the class but I
just make a guess or it’s just my opinion)
- We regret to inform you that some services may be delayed due to the bad weather.
Modals of Possibility and Impossibility (Past Deduction)

To make guess or deduction about the past situation we use past modal of deduction which is
formed by Modal verbs + Past participle.
Form: S+ Modal verb (Must/ Could/ May/ Might) + have+ Past Participle
Must have

When we are almost certain or sure that something was true in the past based on some
evidence and logical reason.
Example:
- Someone broke into our house; you must have left the house unlocked last night. (The house is
in a mess right the only thing we could tell someone entered the house when the door was
unlocked.)
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Sovannaphumi School, TTP
Level 8B
Teacher: Bun Sreyphos
Couldn’t/ Can’t have

When we are almost certain or sure that something was impossible in the past based on some
evidence and logical reason.
Example:
- Peter can’t have passed the exam because he’s in the new class this term. (If he had passed, he
would be in the new class with us so it’s possibly sure that he didn’t make it.)
Might/ May/ Could/ have

We use Might/ May/ Could have to make guess or to say something happening in the past but
you are not sure or there’s no evidence just based on your opinion
Example:
- Did you see my phone? I might have left it somewhere but I can’t find it at all.
 We also use Might/ May/ Could/ + have to talk about the contrast between something was
believe to possibly happen and what actually happened
Example:
- Luckily! We made it on time to the airport yesterday; we could have missed the flight if we left
the house 5 minute later.
- That was stupid to drive that fast. You might have run into pedestrians.
 We also use Might/ May/ Could have to criticize someone’s actions that you actually the
contrast to happen
Example:
- You could have told me about your problem earlier.( But you didn’t tell me and I could not help
you)
- You might have worn something less revealing to meet his parents. ( but you did wear revealing
already and I think it’s wrong to do that)
Alternative Phrases to Modal
-
I’m sure that …
I’m positive that…
I’m certain that…
I’m confident that…
I believe that…
I guess that …
I think …
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Sovannaphumi School, TTP
Level 8B
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Teacher: Bun Sreyphos