Deduction and Speculation

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

For example:

I am waiting for Amanda with another friend,  Danny.
I ask:                   Where is Amanda?

Danny guesses:

  • 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

For example:

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)
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