Modals of Certainty - Must, Can't and Couldn't - Learn English Grammar

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Oxford English Now
Published at : 01 Nov 2020
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Modals of Certainty.

In this tutorial, you will learn 4 general rules about modal verbs and then I will look in detail at the modals of certainty: must, can't and couldn't. With example sentences and common errors analysed you can see how modals can help you sound more like a native. At the end of the video, you have the opportunity to test how much you have learned and understood.

Transcript

Hi, today we are going to learn about modals of certainty. Let's just have a quick refresh on what is a modal.
9 pure modals must, might, may, will, would, can could, shall, should.
1 modals do not change. Ever....
So he must be tired.
Not he musts be tired
This is the same for all pronouns , I must, you must etc.
2. modals are followed by the infinitive without to:
It must be 7am, my alarm is ringing.
Not it must to be 7am...
3. negative modals use NOT or the abbreviation n't.
He can't win, he hasn't trained at all.
Not he don't can win.
4. modals have no past form. So, this links back to point 1, modals don't change. The verbs following the modal show the tense.
So I must have had a bad dream
Not
I musted have had a bad dream.
So let's look in more detail at modals of certainty.
Must
Can't
Couldn't
These are used when you are nearly sure of something being true or not true. It is much more definite than modals of possibility.
So it helps to visualise a line of certainty. On the far left we are 100% sure of a fact in this case, It's cold. the thermometer shows –10°. Modals of certainty come pretty close to this fact. You know it must be cold, you are shivering.
Modals of possibility have far more doubt. It could be cold, it's October it could be cold outside, but it could also be warm and sunny. It’s a possibility.
Then moving onto the negative end of the scale.
Back to our autumn day. It might not be cold, it could be cold. one positive, one negative. a possibility. but not sure.
Then When you very sure that something is not true, you use it can't be cold, because people are swimming in the sea. this is almost certain. and then on the far right. 100% certain of something not being true. It's not cold outside. Thermometer shows the temperature +30°C.

In the present tense
We use must when we are sure something is true.

It must be from Diana because she's in Japan.

When we are sure something is NOT true we use can't/ couldn't (not mustn't)
It can't /couldn't be from Diana because that's not her handwriting.

Be careful when you are sure something is not true, it is not what you would logically think. If you are sure it's true, you say must, so if you are sure it's not true you say Mustn't . No this is only used for modals of prohibition. You mustn't walk on the grass. ( it is against the rules)
When you are sure some thing is not true you use can't or couldn't.

Talking about the past.

We use must have + past participle when we are sure something is true.
Rob went out without his coat, he must have been very cold. ( we are sure this was the case, it is not just possible, you have evidence, he had no coat)

We are sure he did get cold because he didn't take a coat.

When we are sure something is not true in the past.
We use can't /couldn't have + pp
He can't have been cold, because he took a warm coat.
We are sure he didn't get cold because he had the warm coat. Again this is more than just possible, we know he took the warm coat.

Or he couldn't have broken the window, he was at school.

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