Modals of Deduction and Speculation
- 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 Amanda with another friend, Danny.
I ask: Where is Amanda?
- She must be on her way here. (I’m fairly sure this is a good guess)
- She might arrive soon. (maybe)
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:
- He will be at school 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 get 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:
- The cost of living can be high in Paris.
Can is not used to talk about specific possibilities:
- Mom could be in the supermarket (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 Sandra last night?
- She must have forgotten about our appointment.
- She might have worked late.
- She could have lost her way.
- She may have gotten sick.
- 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 package 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 plane 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):
- Labor could be low in the sixteenth century.
This is not used to talk about specific possibilities in the past (instead we use could + have + past participle):
- He could have been working late (not: ‘could be’. As this is a specific possibility, ‘could be’ is present tense)