Key Difference

• “Must have” expresses a strong belief or assumption that something likely happened in the past.
• “Can’t have” expresses a strong belief or assumption that something likely did not happen in the past.

Let’s dive a bit deeper to understand each and see how to use them effectively!

1. Using “Must Have” for Past Deductions

“Must have” is used when we feel almost certain that something happened in the past based on the information we have. Imagine you’re a detective piecing clues together, and the clues make it very likely that something occurred.

• Structure: Subject + must have + past participle.
• Examples:
  • “She must have forgotten our meeting.” (You’re almost certain she forgot.)
  • “They must have left early to avoid the traffic.” (Based on their absence, you deduce they probably left early.)

  Here, you’re making a strong assumption that something did happen because of other clues you’ve observed or know about.

2. Using “Can’t Have” for Past Deductions

“Can’t have” is used when we feel almost certain that something did not happen. This is like concluding that something would be impossible or very unlikely to have occurred given the evidence.

• Structure: Subject + can’t have + past participle.
• Examples:
  • “He can’t have seen the email; otherwise, he would have replied.” (You’re sure he didn’t see it because he hasn’t responded.)
  • “They can’t have arrived yet—the flight only landed an hour ago.” (Given the flight timing, it’s unlikely they’re here yet.)

  So, with “can’t have,” you’re strongly assuming that an event did not happen because of the available evidence.

🕵️‍♀️ Detective Analogy

Imagine you’re a detective investigating a scene. You see an empty coffee cup and an unmade bed:

• Must have: “The person must have been here recently,” because the coffee is still warm.
• Can’t have: “They can’t have left very long ago,” because the bed is still unmade.

📝 Practice Time!

Try these practice sentences to check your understanding. Choose either “must have” or “can’t have”:

1. Sarah ________ (forgot) to bring her ID; that’s why she couldn’t enter the building.
2. They ________ (hear) the news yet; they’re acting completely normal.
3. I ________ (leave) my keys at the restaurant because I still have them with me.
4. John ________ (enjoy) the party last night. He stayed until the end!

✅ Answer Key:

1. Must have (forgot)
2. Can’t have (heard)
3. Can’t have (left)
4. Must have (enjoyed)