100 Mistakes #26 - Must or Should?

On your flight confirmation from Bryanair, it says:

Not your plane

"You must arrive 30 minutes before the flight."

Why, who says?

The airline. This is the airline's point of view.

If you arrive late, they won't let you on the plane. The airline's opinion is pretty important in this context.

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must is a modal verb. But what is a modal verb?

It's a verb that presents points of view.
It is not the language of fact. It is the language of opinion.

Let's look at two modal verbs: must and should

You must arrive an hour before the flight - there is no other option
You should arrive an hour before the flight - this is what I believe is right

The castle must be right behind that hill - there is no other possibility (we don't need to look)
The castle should be right behind that hill - this is what I believe is right (but we need to look)

     must = there is no other option/possibility
     should = I believe that this is right (but there are other options/possibilities)

should is a softer, more flexible version of must.
must is a stricter, exaggerated version of should.
should stresses that this is only a belief. It opens up alternative options.

Many grammar books say that must is obligation or deduction, and should is recommendation or supposition - which is true - but it doesn't show the link between the ideas; it doesn't show what must and should really are in the minds of English speakers.

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Grammar should unite ideas not divide them!
I believe that it is right that grammar unites ideas.

She must be here in a few minutes.
There is no possibility that she won't be here.

She should be here in a few minutes.
I believe that she will be here in a few minutes.

You mustn't talk like that.
Talking like that is not an option.

You shouldn't talk like that.
I believe it is right that you don't talk like that.

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We can also use modal verbs to talk about the past.

     must have = There was no other option/possibility. I believe it happened.
     should have = this is what I now believe was right but it didn't happen.

I saw him on the platform. He must have seen me.
I should have taken the bus.
I saw him on the platform. There is no possibility that he didn't see me.
I now believe it was right to take the bus (but I didn't)

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If someone says that you must do something, and you do something different, you are showing that in reality, there was another option.

For example:

Monday
John says: You must be at the bus station at 8am tomorrow.
There is no other option, from John's point of view.

Tuesday
I arrived at 8:30am. In reality, there was another option.
I should have arrived at 8am. I believe it was right to arrive at 8am.

Not: I must have arrived at 8am.)
must have shows that you believe that you arrived at 8am.
But this is not true. You know you arrived at 8:30am.

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Now you try...


Wearing a uniform is the only option.
You ______ wear a uniform.

I think that it's right that you can take food in your hand luggage.
You ______ be able to take food in your hand luggage.

I think that it's right that you talk to your mum at least once a week.
You ______ talk to your mum at least once a week.

Calling my mum tonight is the only option.
I ______ call my mum tonight.

Do you think it's right to go to the concert tonight?
Do you think I ______ go to the concert tonight?

I think that seeing the new Star Wars film is the only option!
You ______ see the new Star Wars film!

I now think that hitting John was the right thing to do but I didn't hit him.
I ______  ______  hit John.

I believe that I hit John.
I ______ ______ hit John.

I now think that hitting John was not the right thing to do but I hit him.
I ______ ______ hit John. Clue: you will need to use a negative!

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Answers in the comments!