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Talking about possibilities

Modal Verbs of Probability

We can use these modal verbs (also called modals of deduction, speculation or certainty) when we want to make a guess about something. How we choose the verbs is a function of how certain we are.

1: Talking about the present:

2: Using modal verbs to talk about the past:

must / might / could / may / can't + infinitive

Example

I am waiting for Julie with another friend, David.

I ask: 'Where is Julie?'

David guesses:

She must be on the bus. (I'm fairly sure this is a good guess)

She might come soon. (maybe)

She could be lost. (maybe)

She may be in the wrong room. (maybe)

She can't be at home. (I'm fairly sure this isn't true)

Notice that the opposite of 'must' is 'can't in this case.

Will / won't

Should / shouldn't

We use will and won't when we are very sure:

Example

Example

Should and shouldn't are used to make an assumption about what is probably true, if everything is as we expect:

He will dance with us.

She will dance today.

We will all run.

They should already be back.

They should already be walking to the car.

This use of should is not usually used for negative events. Instead, it is better to use will:

Example

The subway will be very busy now (not: 'should be').

Can

Can is used for something that is generally possible, something we know sometimes happens:

Can is not used to talk about specific possibilities:

Example

Example

It can rain in the city.

It can shake.

He could be in my car (not: 'can be').

must / might / could / may / can't + have + past participle

must have + past participle

might / might not have + past participle

could / couldn't have + past participle

may / may not have + past participle

can't have + past participle

Example

You: Where was Julie last night?

David:

She must have forgotten about our date.

She might have worked late.

She could have taken the wrong bus.

She may have felt ill.

She can't have stayed at home.

Will / won't + have + past participle

Should + have + past participle

Could

Will and won't / will not + have + past participle are used for past certainty (compare with present use of 'will'):

Example

Should + have + past participle can be used to make an assumption about something that has probably happened, if everything is as we expect (compare with present use of 'should' ):

My aunt will have arrived earlier in Australia.

Andrea will have given birth before.

We can use could + infinitive to talk about a general possibility in the past (compare with the use of 'can'):

This is not used to talk about specific possibilites in the past (instead we use could + have + past participle):

Example

Example

Example

My aunt should have finished by now.

Darla should have left by now.

The supermarket could be cheaper in 1990.

Fashion could be more valuable in 1500.

Religion could be very important in the 15th century.

He could have been working late (not: 'could be'. As this is a specific possiblity, 'could be' is present tense)