Use must, might or can't + be + the -ing form to guess about an action happening right now.
Guessing about right now
Hiroshi: Maya isn't answering her phone.
Camila: She might be cycling home.
Hiroshi: No, her bike's still outside the design building. She must be working.
Try it: Farid: Why is the music room locked? Yasmin: The choir ___. I can hear them singing. Choose the words for the blank. (must be practicing / might be practicing / can't be practicing) Answer: must be practicing — She can hear the action, so she is sure: use must be + the -ing form.
modal + be + the -ing form
The form is the modal + be + the -ing form: must be working, could be waiting.
Ethan: Where's Lani? Her coat's on her chair.
Sofia: She could be talking to someone downstairs.
Try it: Diego: Somebody is hammering upstairs in the library. Build Sofia's guess. Leave out the word you do not need. Answer: They must be fixing something. — A guess about an action now needs be + the -ing form: must be fixing.
Keep the be
Don't drop the be: the modal, be and the -ing form all stay.
Lars: Ira's been gone for an hour.
Mateo: She could be sketching somewhere. She loses track of time then.
Not "She must working now." — say "She must be working now.". Keep be in the middle.
Try it: Wei: Diego's car isn't in its usual spot. Build Amina's guess. Leave out the word you do not need. Answer: He might be driving. — Keep be, then the -ing form: might be driving.
Tired, or resting? state vs action
Use must be + -ing to guess about an action in progress; for a state (tired, hungry), use must be + an adjective.
Yasmin: Matilda's been at the clinic since six this morning.
Liam: She must be hungry. She must be finishing her last appointment now.
Try it: Amina: I can hear someone moving boxes in the storeroom. Hiroshi: Ethan ___. He said he'd sort it out today. Choose the words for the blank. (must be tidying / must be tidy / must tidying) Answer: must be tidying — You can hear an action going on now, so use be + the -ing form: must be tidying.