To build a past action done to something, put have been and the action word right after the helper word. Put not after the helper word to say it did not happen. Start with the helper word when asking a question.

🔹 The maintenance records should not have been left unsigned before the flight.

🔹 Could the lapse have been found earlier by the investigator during the inquiry?

Use should have been or ought to have been to point out a duty that was missed. Use could have been to show something was possible but did not happen.

🔹 The loose bolts ought to have been tightened during routine maintenance.

🔹 The wreckage could have been moved before the storm hit the crash site.

Use must have been when you are certain about a past event from clues. Words like might or may show a possible guess, while cannot have been means it was impossible.

🔹 The flight recorder must have been switched off before the collision, as no audio exists.

🔹 Key findings of the inquest might have been altered before being presented.

Use words like is believed or are said followed by to have been to report what people think today about a past event. Match the first word to one or many items.

🔹 Two earlier warnings are said to have been ignored by the chief investigator.

🔹 The disaster is believed to have been caused by severe engine damage.