We can use these modal verbs (also called modals of deduction, speculation or certainty) when we want to make a guess about something. We choose the verb depending on how sure we are.
1: Talking about the present:
must / might / could / may / can’t + infinitive
I am waiting for Julie with another friend, David.
I ask: ‘Where is Julie?’
- 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
We use will and won’t when we are very sure:
- She’ll be at work now.
Should / shouldn’t
Should and shouldn’t are used to make an assumption about what is probably true, if everything is as we expect:
- They should be there by now.
- It shouldn’t take long to drive here.
This use of should isn’t usually used for negative events. Instead, it’s a better idea to use will:
- The underground will be very busy now (not: ‘should be’).
Can is used for something that is generally possible, something we know sometimes happens:
- Prices can be high in London.
Can is not used to talk about specific possibilities:
- He could be on the bus (not: ‘can be’).
2: Using modal verbs to talk about the past:
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
You: Where was Julie last night?
- 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
Will and won’t / will not + have + past participle are used for past certainty (compare with present use of ‘will’ above):
- The parcel will have arrived before now.
Should + have + past participle
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’ above):
- The train should have left by now
We can use could + infinitive to talk about a general possibility in the past (compare with the use of ‘can’ above):
- Prices could be high in the sixteenth century.
This is not used to talk about specific possibilites in the past (instead we use could + have + past participle):
- He could have been working late (not: ‘could be’. As this is a specific possiblity, ‘could be’ is present tense)
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