must have been doing (guessing about the past) (B2 Grammar)

You use these guesses when you don't know for sure — so the modal you pick shows how sure you are. Read it as a scale of certainty: - must have been — you're sure it's true (the evidence really only points one way). - might / could / may have been — it's just possible, one option among several. You're guessing. - can't have been — you're sure it's NOT true (the evidence rules it out). Same evidence can give different guesses — choose the modal that matches your confidence. The same scale works for a guess about right now (must be / might be / can't be) and about the past (must have been / might have been / can't have been). Register: may is a bit more formal than might or could; the three mean the same here.

How sure are you? must → might/could/may → can't

• Her boots were caked in mud. → She must have been walking in the fields. (sure it's true)
• He didn't pick up. → He might have been driving. (possible — maybe, maybe not)
• I saw her at her desk all afternoon. → She can't have been shopping in town. (sure it's not true)
• Not "He didn't answer, so he must have been ignoring me." — say "He didn't answer, so he might have been driving.". Don't jump to must (sure) when the evidence only makes something possible — use might / could / may for a guess you're not certain about.

must / might + have been + the -ing form

To guess about something that was going on at a past moment, use a modal + have been + the -ing form. It puts the spotlight on an action in progress, not a finished one.

• She looks exhausted. → She must have been working all night.
• No answer at ten. → They might have been sleeping.
• His clothes were muddy. → He could have been playing football.
• Not "She must have been work all night." — say "She must have been working all night.". After have been, use the -ing form: have been working, not "have been work".
• Try it: Lars's hair was wet when he came in. Guess what was going on. (Leave out the word you don't need.) Answer: He must have been swimming. — To guess about an action in progress in the past, use must have been + the -ing form: He must have been swimming. "Swim" is the base verb and breaks the pattern, so leave it out. After have been comes the -ing form.

Don't drop the have

The full shape is modal + have + been + -ing. The little have is easy to lose in speech, but it has to be there.

• might been sleeping → He might have been sleeping when I called.
• must been waiting → They must have been waiting a long time.
• could been studying → She could have been studying all evening.
• Not "He might been sleeping when I called." — say "He might have been sleeping when I called.". Keep the have: might have been sleeping, not "might been sleeping".
• Try it: The line was busy for an hour. Guess why. Build the full form. Don't drop the have. (Leave out the word you don't need.) Answer: She might have been talking to a friend. — The full form keeps the have: She might have been talking. "Might been" drops the have, which the form always needs, so leave it out. It's modal + have + been + the -ing form.
• Try it: The streets were soaked and there were puddles everywhere. You're sure — the evidence only points one way. Pick the modal that shows you're certain it's true. (Leave out the words you don't need.) Answer: It must have been raining all night. — The evidence points one way and you're sure, so use must have been — the 'sure it's true' modal: It must have been raining all night. "Might have been" only means it was possible, and "can't have been" means it was impossible, so both are the wrong level of certainty here.

can't have been + -ing (ruling it out)

At the 'sure it's NOT true' end of the scale, when the evidence makes a past action impossible, use can't have been + the -ing form. Keep the have here too — can't have been, never "can't been".

• You heard him typing the whole time. → He can't have been sleeping.
• She was on stage all evening. → She can't have been waiting outside.
• The car never moved. → They can't have been driving anywhere.
• Not "He can't been sleeping — I heard him typing." — say "He can't have been sleeping — I heard him typing.". The negative keeps the have: can't have been sleeping, not "can't been sleeping".
• Try it: You heard him typing the whole time. Say it was impossible, based on the evidence. (Leave out the word you don't need.) Answer: He can't have been sleeping. — To say it was impossible based on what you noticed, use can't have been + the -ing form: He can't have been sleeping. "Can't been" drops the have, which this form still needs, so leave it out. The negative keeps have too.