must be doing (modal + continuous)
must be doing (modal + continuous) means “Modal + be + V-ing: guessing about what is happening right now”. To guess about an action in progress now, use a modal + be + V-ing: He must be sleeping. Formation: modal + be + V-ing
Modal + be + V-ing: guessing about what is happening right now
B2Formation
modal + be + V-ing
Explanation
To guess about an action in progress now, use a modal + be + V-ing: He must be sleeping. The V-ing keeps the continuous meaning (happening at this moment), and the modal shows your certainty: must (I'm sure), might, may or could (maybe), can't (surely not). It answers 'What is probably going on right now?' from the evidence you can see.
Examples
Note
This is deduction, not the present continuous of arrangement. Don't confuse must be doing (a guess about now) with the plain be doing that describes a fixed future plan (I'm meeting him tomorrow).
FAQ
must be doing (modal + continuous) — what does it mean and how is it used?
Modal + be + V-ing: guessing about what is happening right now. To guess about an action in progress now, use a modal + be + V-ing: He must be sleeping. The V-ing keeps the continuous meaning (happening at this moment), and the modal shows your certainty: must (I'm sure), might, may or could (maybe), can't (surely not). It answers 'What is probably going on right now?' from the evidence you can see.
must be doing (modal + continuous) — how do you form it?
modal + be + V-ing
must be doing (modal + continuous) — any usage notes?
This is deduction, not the present continuous of arrangement. Don't confuse must be doing (a guess about now) with the plain be doing that describes a fixed future plan (I'm meeting him tomorrow).