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Probability

Level: beginner

Possibility

We use may, might and could to say that something is possible, but not certain:

They may come by car. (= Maybe they will come by car.)
They might be at home. (= Maybe they are at home.)
If we don't hurry, we could be late. (= Maybe we will be late.)

We use can to make general statements about what is possible:

It can be very cold here in winter. (= It is sometimes very cold here in winter.)
You can easily get lost in this town. (= People often get lost in this town.)

Be careful!

We do not use can to talk about specific events:

A: Where's John?
B: I'm not sure. He may/might/could be 
(NOT can) in his office.

Notice the difference in meaning between can and may/might/could:

That dog can be dangerous.
(= Sometimes that dog is dangerous. I know.)

That dog may/might/could be dangerous.
(= Perhaps that dog is dangerous. I don't know.)

can and may/might/could

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Level: intermediate

We use may have, might have or could have to make guesses about the past:

I haven't received your letter. It may have got lost in the post.
It's ten o'clock. They might have arrived by now.
Where are they? They could have got lost.

We use could to make general statements about the past:

It could be very cold there in winter. (= It was sometimes very cold there in winter.)
You could easily get lost in that town. (= People often got lost in that town.)

could and could have

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Impossibility

Level: beginner

We use can't or cannot to say that something is impossible:

That can't be true.
You cannot be serious.

Level: intermediate

We use can't have or couldn't have to say that a past event was impossible:

They know the way here. They can't have got lost!
If Jones was at work until six, he couldn't have done the murder.

Certainty

Level: beginner

We use must to show we are sure something is true and we have reasons for our belief:

It's getting dark. It must be quite late.
You haven’t eaten all day. You must be hungry.

We use should to suggest something is true and we have reasons for our suggestion:

Ask Miranda. She should know.
It's nearly six o'clock. They should arrive soon.

Level: intermediate

We use must have and should have for the past:

They hadn't eaten all day. They must have been hungry.
You look happy. You must have heard the good news.
It's nearly eleven o'clock. They should have arrived by now.

Probability 1

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Probability 2

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Probability 3

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Probability 4

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Probability 5

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Comments

Dear teahcers,

1- Why can't we use " can have " to make guesses about the past like may have, might have or could have ?

2- Why can't we use " couldn't " to say that something is impossible like " can't " ?

Hello ahmed16,

I wish that it were a bit more logical, but I'm afraid this is just the way that English has developed as people have been speaking it over the last several centuries. There are some technical analyses of modal verbs that can help understand how they work a bit better, but I'm afraid they require a bit of study to make sense of and are well beyond the scope of what we do here.

Best wishes,

Kirk

The LearnEnglish Team

ok
Do you mean that it is a language and its rules must be adhered to, even if what I am saying is relatively true?

Hello ahmed16,

If you want to be understood by others, then yes, you generally have to follow the rules that other people follow when speaking. There are ways to break the rules and still be understood, but in my opinion it's important to learn the rules most people use first.

Hope this helps.

All the best,

Kirk

The LearnEnglish Team

Dear teachers
can we use " can have " to make guesses about the past like may have, might have or could have ?

A second question
can we use " couldn't " to say that something is impossible ?

Hi ahmed16,

Good questions! I'll answer them in order.

  1. No. We can't use can have, but we can use the negative form can't have to show that we think it's not possible that something happened. Have a look at this page on Modals for deductions about the past for more examples and explanation.
  2. We can use couldn't have to say that something was impossible (in the past), but we can't use couldn't on its own with this meaning. See this page on can and could for more information.

Does that make sense?

Jonathan

The LearnEnglish Team

Coul you help me, please? Can we use OUGHT TO instead of SHOULD in these particular cases?
1. They insisted that we should have dinner together.
2. It is essential that everyone should be here on time.
3. It is strange that he should be late. He is usually on time.
4. If it should rain, take an umbrella with you.
5. It is cold outside. I should wear a coat (like advice).

Hello iamsashafierce,

We don't use ought to in place of should in conditional structures (#4) or as an alternative subjunctive form (#1, 2, 3), so only in the last sentence (#5) can you replace should with ought to.

 

Peter

The LearnEnglish Team

This tip is really interesting. Thanks.

I reluctantly agreed to a postponement on condition that the sale should be completed and the boat handed over by 31st August.

You may borrow the book on condition that you return it tonight

Teacher, why is "should" used in 1st sentence but not in 2nd sentence?

Thanks for your reply in advance.

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