Can't/Must + verb be "probability"

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Senior Member
Arabic- Egyptian

Can't and must as modals of probability - Are they only used with verb to be?

I noticed that when we use them with other verbs, they're no longer modals of probability.

She can't be on the bus. /She can't do her homework.

The first one is a modal of probability and the second one is of ability, so the meaning changed.

However, I noticed that maybe if we use the continuous form of the verb, it works.

She can't be doing her homework. I saw her watching TV.

So,are they only used with verb to be or verb to be +verb + ing?

My guess is yes. This is because "must" and "can't" when used as present modals of probability, we're either talking about someone "being something/somewhere" or someone "be doing something" at the moment of speaking. So, more or less verb to be is there. I can't think of any example where we don't use verb to be.

Is my guess correct?

Thank you
  • velisarius

    Senior Member
    British English (Sussex)
    Don't forget the present perfect: She can't have done her homework: she's been watching TV all evening.
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