"Must (Probability)" has this structure: "must have PP". But can "Must (Obligation)" have that structure?

Ok, now, must (Probability) can be used to express the high probability

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

& in the past: She must have been on the bus yesterday. (I'm fairly sure this is a good guess)

But, must (obligation) can be used to express strong obligation

Ex: She must study today.

& in the past: She had to study yesterday (as in textbook)

My question is that:

Can "Must (Obligation)" have the structure "must have PP"?

So, is "She had to study yesterday" (as in textbook) the same as "She must have studied yesterday"?

Or, whenever we say "She must have studied yesterday", people will think "must" here refers to a Probability?

  • 1
    They'll think it refers to probability. The only way to use must to describe past obligation is with a "framing" construction, one that will sound somewhat dated and awkward in today's English. One example: "It was clear that he must study yesterday." Another: "Nothing would suffice then but that he must give his opinion on the matter."
    – Robusto
    Jan 22 '17 at 15:17
  • 3
    In your second link, if you scroll all the way down, you'll see the answer to your question: "Remember 'must have done' is a modal verb of deduction or speculation, not obligation in the past. For example: Julie must have left. Her coat's not here." Jan 23 '17 at 8:24

The example can mean both things, it very much depends on the context provided. Most of the time it will refer to probability and I would personally prefer not to use "must" for the other meaning but it can be done.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.