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Structure: ‘must have’ & ‘can’t have’

This time in our structure help we talk about using ‘must have’ and ‘can’t have’. When we think about these two modals we think about them in relation to obligations for must, and ability for can. However, as with all modal verbs, there is more than one meaning. In addition to the above uses, we can use these modals in relation to probability / certainty. Both of these expressions relate to the past. ‘Must have’ expresses a high level of certainly that something was true or that it happened. Look at the following examples.

He is more than an hour late, he must have forgotten the takeoff time.
They arrived at the hotel after 2am, they must have been exhausted.

In both of these examples the speaker expresses a high level of certainty about what is said by choosing to use ‘must have’ (+ past participle). If the speaker isn’t so such, there are other expressions the can be used, however, the choice of expression here indicates a high level of certainty to the listener(s).

‘Can’t have’ (+ past participle) expresses a high level of certainty that something wasn’t true or didn’t happen. In this sense our two expressions represent opposites to each other. Look at the following examples.

His result in the test was bad, he can’t have studied for it.
They almost hit the animal while landing, they can’t have seen it until the last second.

Similarly in this case, the expression chosen indicates a high level of certainty that something wasn’t true or didn’t happen, other expressions exist to indicate different levels of certainty.

Now try to write some of your own examples using these two expressions.

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Structure: ‘Modals 8’

Modals and obligation

In the diagram below we can see that both should and must can be used to talk about present or future obligations. Again, when we want to talk about obligations in other tenses we can use other verbs which aren’t modals. The most common one is have to. Use have to in the past and with would, will or other modals (remember we never use two modals together). Review the examples below the diagram and tray to write some examples yourself. Then try to used have to for obligations in other tenses.

How to talk about obligations.
How to talk about obligations.

Present
He should wait before entering the plane. (not a strong obligation)
They must sit during the takeoff and landing. (strong obligation)

Future
You should get a taxi from the airport after you disembark.
She must file the report before 8pm tomorrow evening.

Past
They had to divert due to weather conditions. (strong obligation)
We had to renew our medical licence before returning to the line. (strong obligation)

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Structure: ‘Modals 7’

Modals and prohibition

From the diagram, we can see that to talk about prohibition there are five options, depending on the situation. ‘Mustn’t’ is the strongest structure, while ‘shouldn’t’ is the weakest. ‘Can’t’ and ‘may not’ relate to medium levels of prohibition. Look at the diagram and then look at the examples below for some more information.

How to talk about prohibition.
How to talk about prohibition.

Present
Passengers can’t enter the cockpit during flights.
You may not put your bag under the seat in a beside and emergency exit.
You shouldn’t drink your own alcohol on board.
You mustn’t smoke on board.

Future
We can’t have a day off next Monday.
They may not land after 2am, the airport will be closed.
He shouldn’t perform the landing for the next flight, it’s not his turn.
They mustn’t switch seats after the planes takes off.

Reported speech
He told us we couldn’t switch our flight.

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Structure: ‘Modals 6’

Modals and permission

Looking at our diagram we can see that to ask for permission in the present or future we have four options; can, could, may or might. Could is a little more polite than can, and may or might are very formal, so choose the correct model for your situation.

Asking for permission.
Asking for permission.

Present
Can I open this window shade?
Could I open this window shade?
May I open this window shade?
Might I open this window shade?

Future
Can I use the jumpseat on the next flight?
Could I use the jumpseat on the next flight?
May I use the jumpseat on the next flight?
Might I use the jumpseat on the next flight?

To refer to permission in the past use ‘allow sb to’ (which is not a modal verb).

eg The company allowed me to continue flying 48 hours after the incident.

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Structure: ‘Modals 5’

Time references for modal verbs

Adding to what we saw previously, we look at the time reference for our modal verbs in each of their uses. Looking at our diagram below, we can see that to talk about ability in the past we use ‘could’ and in the present we use ‘can’.

Ability in the future?

Then how do we talk about ability in the future? Well, in the future we use ‘be able to’.

eg. By next year I’ll be able to fly three single engine aircraft.

‘Be able to’ isn’t a modal verb so it isn’t in our list, but we can use it with a similar meaning to a modal verb with time references unavailable to our modal verbs (i.e. not just future, but also with present perfect, after ‘would’ etc…).

Time reference for modal verbs.
Time reference for modal verbs.

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Structure: ‘Modals 4’

Modals related to ability, permission, prohibition and obligation

We talked previously about how our modal verbs related to probability. Next we look at specific meanings to each of our modal verbs. Follow the examples and write some of your own to practice.

Ability

Ability
– Can you fly a multiengine plane?
– I could fly a glider when I was 17.

Permission

Permission
– Can we use runway 11 to land?
– The controller said that they could begin their startup.
– You may use the lavatory when the seatbelt sign is switched off.
– Might I switch seats? This one doesn’t recline.

Prohibition

Prohibition
– You can’t sit in the emergency exit seat with a baby.
– They told us that we couldn’t board the plane first.
– You may not bring liquids of more than 100ml on the flight.
– You shouldn’t drink a lot of alcohol on a flight.
– You mustn’t smoke on a flight.

Obligation

Obligation
– He should review the weather charts before the flight.
– Passengers must sit in their assigned seats.

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