We can use “might have” and “must have” when we are making deductions about the past.
Use “might have” (or “may have”) + the past participle when you are not totally certain of something. Like:
- I’m not totally sure, but I might have failed the test.
- Historians aren’t certain, but the silver coin found in Maine may have been left there by Viking explorers.
Use “must have” + the past participle when you are more certain. Like:
- The test was extremely difficult. I must have failed it.
- There is no other explanation. The silver coin found in Maine must have been left there by Viking explorers.
The British Council has a great article about this topic!