Use passive voice with modal verbs: MODAL + BE + past participle. The modal (can, should, must, etc.) comes before BE, which stays in base form.
Quick tip: Modal Passive = MODAL + BE (base form) + past participle. Can be, should be, must be, might be + past participle!
Form: MODAL + BE + past participle
Modal verbs (can, should, must, might, etc.) + BE (base form) + past participle. BE is always in base form after modals.
It can be done. (ability - passive)
It should be finished. (advice - passive)
It must be completed. (obligation - passive)
It might be delayed. (possibility - passive)
Common modal passives
Can be done (ability), should be done (advice), must be done (obligation), might be done (possibility), has to be done (necessity), needs to be done (necessity).
This problem can be solved. (it's possible to solve)
The work should be finished today. (advice/expectation)
The door must be locked. (obligation/necessity)
It might be cancelled. (possibility)
Common mistakes
Not "It can done." — say "It can be done.". Modal passive needs BE between modal and past participle. Can be done (not can done)..
Not "It should is finished." — say "It should be finished.". After modal, use BE (base form), not is/are/was. Should be (not should is)..
Practise it
Modal + BE (base form) + past participle. Can be, should be, must be, might be + past participle!