Basics. Past Modals of Deduction: correct answers
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When we want to make a guess or a logical conclusion about something that happened in the past, we use past modals of deduction. For example, if your friend looks exhausted, you might say, "You must have stayed up late." If a suspect has a solid alibi, a detective would deduce, "They can't have committed the crime."
In this challenge, you will step into the shoes of an amateur detective to solve everyday mysteries, from missing pizza slices and muddy dogs to locked-room puzzles and ancient Egyptian tombs. You will practice evaluating evidence to choose the correct modal verbs, using must have for logical certainties, might/may/could have for possibilities, and can't/couldn't have for logical impossibilities.
You'll work through 15 questions featuring a variety of single-choice, multi-choice, drop-down, and drag-and-drop formats to test your deductive reasoning.
Try the quiz to check your knowledge!
Correct answers
Question 1
The correct answers are could have and can't have.
Could have expresses a past possibility. Because he wasn't wearing glasses, it's very possible he made a mistake.
Can't have expresses a strong negative deduction. The tractor tracks provide undeniable proof that aliens were not responsible for the crop circle.
Question 2
Help the amateur detective piece together the mystery of the missing chocolate cake. Choose the correct modal verb for each gap to complete her notes.
I left the cake on the counter, and now it's completely gone! Someone must have taken it while I was outside watering the plants.
It couldn't have been my little brother, because he is currently at a summer camp in another state.
The dog might have eaten it, but he looks completely innocent right now, and there's no frosting on his nose.
Someone must have taken it while I was outside watering the plants.
We use must have + past participle when we are almost certain that something happened in the past based on the evidence (the cake is gone).
It couldn't have been my little brother, because he is currently at a summer camp in another state.
We use couldn't have or can't have + past participle when we are almost certain that something did NOT happen in the past (it's physically impossible since he is out of state).
The dog might have eaten it, but he looks completely innocent right now.
We use might have + past participle when we think it is possible that something happened, but we aren't completely sure.
Question 3
Help the amateur detective solve the Great Pizza Mystery by choosing the correct deduction.
Liam _____ the last slice of pizza because he was visiting his grandmother in another city all weekend!
The correct answer is can't have eaten.
We use can't have + past participle to express strong certainty that something in the past was impossible. Since Liam was in another city, it is logically impossible for him to be the pizza thief!
Must have expresses certainty that something did happen, while might have only shows a possibility. Shouldn't have is used for regrets or criticism, not deduction.
Question 4
The correct answers are can't have and must have.
We use can't have + past participle when we are almost certain that something did not happen in the past (it's logically impossible because the window was locked).
We use must have + past participle when we are almost certain that something did happen based on strong evidence (the muddy paw prints).
Question 5
Help the forgetful professor retrace his steps by picking the most logical phrase.
I suppose I _____ my glasses in the staff room, but let me check my coat pockets just in case.
The correct answer is might have left.
We use might have (or may have / could have) + past participle to express that something was possible in the past, but we aren't completely sure. The phrase "just in case" shows that the professor is still uncertain.
If he used must have left, he would be absolutely certain and wouldn't need to check his pockets!
Question 6
The correct answers are The new guy might have eaten it, The new guy could have eaten it, and The new guy may have eaten it.
To express that something was possible in the past, we use might have, may have, or could have followed by a past participle.
"Must have eaten it" expresses a strong certainty, not just a possibility. "Should have eaten it" expresses a regret or a missed good idea, rather than a deduction!
Question 7
Read the texts between two confused friends at an empty concert venue. Choose the appropriate modal expression for each gap to complete their conversation.
Friend A: Where is everyone? The stage is completely empty. The band must have finished their set early and gone home!
Friend B: They couldn't have played yet. The tickets say the show starts at 9:00 PM, and it's only 8:30 PM right now!
Friend A: Oh no... do you think we might have come to the wrong venue?
Friend B: Let me check the ticket. Oh, oops. The concert is tomorrow.
The band must have finished their set early and gone home!
Friend A sees an empty stage and makes a strong logical deduction that the concert is over (must have + past participle).
They couldn't have played yet.
Friend B looks at the time and realizes it is impossible for the band to have played already (couldn't have + past participle).
...do you think we might have come to the wrong venue?
Friend A is guessing about a possible past mistake (might have + past participle).
Question 8
The correct answers are couldn't have won and can't have won.
When we are strongly certain that something did NOT happen in the past, we use can't have or couldn't have + past participle.
"Might not have won" only expresses a possibility. "Shouldn't have won" means it was a bad idea or unfair, but doesn't mean it was impossible!
Question 9
The correct answers are couldn't have and must have.
Couldn't have is used for a strong negative deduction in the past: it is historically impossible for ancient Egyptians to have invented digital watches.
Must have expresses a strong positive deduction: since Professor Jones was the only person there, it is the only logical conclusion that he dropped it.
Question 10
Complete the traveler's confused diary entry about a strange photo from their vacation. Choose the correct past modal phrase for each blank.
I just found a picture of myself wearing a giant, neon sombrero in Madrid. I must have bought it at that street market, because I definitely didn't pack one in my suitcase!
However, I can't have paid a lot of money for it, because I still had almost all my cash the next morning.
My friends might have dared me to wear it as a joke, but honestly, my memory of that evening is a bit fuzzy!
I must have bought it at that street market...
Since the traveler didn't pack the hat but is wearing it in the photo, it is a logical certainty that they bought it (must have + past participle).
However, I can't have paid a lot of money for it...
Because the traveler still had their cash, it is a logical impossibility that the hat was expensive (can't have + past participle).
My friends might have dared me to wear it...
The traveler's memory is fuzzy, so this is just a possibility, not a certainty (might have + past participle).
Question 11
The correct answers are couldn't have escaped and can't have escaped.
To deduce that something was impossible in the past, standard English uses can't have or couldn't have + past participle.
"Mustn't have" is generally avoided for logical impossibility in standard grammar (we prefer can't/couldn't have). "Had not to escape" is grammatically incorrect.
Question 12
The correct answers are can't have and might have.
Can't have is correct for the first blank because the presence of her coat and keys makes it logically impossible that she has already left.
Might have is correct for the second blank because it expresses a 50% possibility in the past: Sarah isn't sure if her roommate turned on the TV or if the cat made it warm.
Question 13
The correct answers are He must have jumped into the neighbor's pool and He might have chased a frog into the pond.
Must have + past participle correctly expresses strong certainty about the past, while might have + past participle correctly expresses a past possibility.
"Can have" is grammatically incorrect for affirmative past deductions (we use could have instead). "Must has" is incorrect because modal verbs must always be followed by the base form of the verb (have), never has.
Question 14
Finish the terrified babysitter's text message to her best friend.
The front door is locked from the inside, so the pizza delivery guy _____ the one making those spooky noises upstairs!
The correct answer is couldn't have been.
We use couldn't have (or can't have) + past participle to deduce that a past event was impossible. Because the door is locked from the inside, the babysitter logically deduces that the delivery guy cannot possibly be inside the house.
Must have been would mean she is certain he is upstairs, which contradicts the locked door.
Question 15
Complete the dog owner's realization as she looks at her very messy pet.
Look at Buster's muddy paws! He _____ through the neighbor's prize-winning petunia bed again.
The correct answer is must have dug.
We use must have + past participle to show we are almost 100% sure that something happened in the past based on present evidence. The muddy paws are the "clue" that leads to this strong deduction.
Can't have means it's impossible, which contradicts the mud. Should have is for advice or regret, and might dig refers to a present or future possibility.