Must have (done) – (1) – to express the present supposition /guessing of what would have happened in the past.
- We must have taken the wrong road. The sign board was not clear.
- She predicted that such a creature must have existed.
- It must have been dreadfully difficult. How did you manage the situation?
- It must have been agonising for the people around you.
- We rang the calling bell but he didn’t come out. He must have been asleep.
- You looked weak and pale. You must have been very tired of the heavy work.( i.e I am sure you were very tired)
Must have (done) (2) – to show that it is necessary for the first thing to have happened before the second thing can happen.
- In order to qualify for the full pension benefits in the Bank, you must have completed 33 years of confirmed service.
- If you are to be included in the list of eligible candidates for the benefits, the application must have been submitted before 31st of December.