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 5

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Comments

I had the same question, so thanks everyone!

Thanks a lot for the valuable answer to my question sir.
I have got another question that some of the people and websites consider "ought to" and "need to" as modal verbs. Are these indeed modal verbs sir?

Hell Vijay Soni,

These are examples of what are called 'semi-modals'. In some aspects they function as modals but in others they are more like 'normal' verbs. For example, modals are usually followed by the bare infinitive (without 'to'), but these verbs have 'to'. Modals do not have regular past forms but rather perfect forms (should > should have), whereas 'need to' has a past form 'needed to'. On the other hand, modals do not have a different third person form (I should > He should), and this is true of 'ought to' (I ought to > He ought to).

There are other examples of semi-modals, such as 'dare'.

 

Best wishes,

Peter

The LearnEnglish Team

Hi, teachers,
It’s ten o’clock. They might have arrived now.
Does this sentence suggest that it's a hypothetical situation? meaning that it was possible for them to arrive if they left ,but they didn't??
They could have arrived hours ago.
Does this sentence have two meanings?
The first is hypothetical like the previous sentence ,and the second is about expressing possibility of a past situation ?
I hope that my questions are clear.

Hello Inas,

In your first pair of sentences, the second sentence is hypothetical in one sense, i.e. it's possible they have arrived, but also possibly they haven't – we don't know. But this kind of hypothetical situation is a bit different from the hypothetical situations in, for example, third conditionals.

Regarding the other sentence, which two meanings do you have in mind? I'm afraid I'm not sure I understand what your questions are really about. If there's some specific context, it would be helpful to know that.

All the best,
Kirk
The LearnEnglish Team

 

Hi, Kirk,
Thanks for your reply.
They could have arrived hours ago.
I mean that if this sentence refers to a possibility of their arrival but in the past tense or it means that all the situation is hypothetical as they didn't go somewhere to arrive, but they could have arrived if they left.
I hope it's clearer now.
Inas

Hello Inas,

Thanks for clarifying that. Yes, it could be used in both of those ways.

All the best,
Kirk
The LearnEnglish Team

Hi, Kirk,
It’s ten o’clock. They might have arrived now.
Could this sentence used in the same both ways??
Thanks,
Inas

Hello Inas,

You could use it in both ways, though if you wanted to communicate the hypothetical meaning, you'd have to clarify this in the context for this hypothetical meaning to come across.

All the best,
Kirk
The LearnEnglish Team

what is the difference between i might come and i might not come? when should i use might not?

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