logo
logoGrammar Error
Test your EnglishLearnChallengesArticlesAbout
logoGrammar Error

Basics. Present Modals of Deduction: Must, Can't, and Might: correct answers

StartTo challengesDownload PDF
Jump to answers ↓

Present Modals of Deduction: Must, Can't, and Might

How do you express certainty or make a logical guess in English? We use present modals of deduction to show how sure we are about a situation based on the available evidence. For example, if someone is yawning constantly, you might deduce, "You must be exhausted!" If you are looking for your keys, you might guess, "They could be in my jacket pocket." Conversely, if a friend claims to have seen a dinosaur, you'd probably respond, "That can't be true!"

This challenge tests your ability to choose the correct modal verb based on context clues. You will practice using must for strong logical certainty, can't for logical impossibilities, and might, may, or could for 50% possibilities. You will also explore negative possibilities using might not and may not. The scenarios cover fun everyday mysteries, from figuring out who stole the office chocolate cake to deducing what is inside an oddly shaped package left on your porch.

You will work through 15 questions in single-choice, multi-choice, drop-down, and drag-and-drop formats to hone your deductive reasoning.

Try the quiz to check your knowledge!

MediumB1 | IntermediateB1Auxiliary verbInfinitiveModal verbNegationPresent tenseVerbVerb moodProgressive tenseSimple tense

Correct answers

Question 1

Become the office gossip for a moment! Select ALL the sentences that use modals of deduction correctly.
"Look at Bob wearing a full tuxedo to the office on a Tuesday! ..."

The correct answers are He must be going to a special event later. and He could be attending a wedding this afternoon.

Must is used for strong positive deductions, and could is used for possibilities. Both are perfectly correct here!

"He can be a secret agent" is incorrect because can is not used to guess about specific present situations.

"He must to have" is incorrect because modal verbs are always followed by a bare infinitive (without "to").

Question 2

Complete the roommates' conversation about their very weird cat by choosing the correct modal verbs of deduction.
Roommate A: "Why is Whiskers staring at the blank wall?" Roommate B: "I don't know. There _________________________ be a tiny bug crawling there that we can't see." Roommate A: "No way, the wall is perfectly clean. She _________________________ be listening to mice in the pipes!" Roommate B: "Well, she's purring happily and rubbing her face on the floor, so she _________________________ be scared of whatever it is."

could

Could expresses a possibility. Roommate B is guessing about a bug but isn't sure.

must

Must shows strong certainty based on evidence. Since the wall is clean, Roommate A concludes that hearing mice is the only logical explanation.

can't

Can't shows strong certainty that something is false. Because the cat is purring happily, it is impossible that she is scared.

Question 3

Help the frantic cat owner brainstorm! Select ALL the options that correctly complete the sentence to show a 50% possibility.
"I can't find Mr. Whiskers anywhere! I'm not entirely sure, but he _____ hiding in the laundry basket again."

The correct answers are might be, could be, and may be.

When we are guessing about a present situation and think it is possible (around 50% certainty), we use might, may, or could followed by the base verb.

Must be is incorrect here because it implies you are almost 100% certain, which contradicts "I'm not entirely sure."

Can be is generally not used for specific present deductions; it is used for general truths (e.g., "Cats can be very sneaky").

Question 4

Fill in the blanks as two friends try to figure out why their buddy is late for dinner.
"Leo is half an hour late. His phone is going straight to voicemail, so his battery _________________________ be completely dead.
Or, he _________________________ be stuck in that terrible construction traffic on 5th Avenue.
He _________________________ be at home sleeping, though. He texted me that he was leaving his house an hour ago!"

must

Phones going straight to voicemail is strong evidence, leading to the logical deduction that the battery must be dead.

might

Traffic is a reasonable guess, but they don't have proof, so might shows possibility.

can't

Since he texted that he was leaving an hour ago, it is logically impossible that he is still at home sleeping. Therefore, he can't be at home.

Question 5

You and your friends hear a knock at the door. Select ALL the sentences that make a correct logical deduction.
Context: You ordered a pizza 10 minutes ago, but the restaurant said delivery takes 45 minutes. Your best friend Sarah promised to arrive at exactly 8:00 PM. It is now 8:00 PM, and someone is knocking.

The correct answers are It must be Sarah. and It can't be the pizza delivery.

When we are almost 100% sure something is true based on evidence, we use must (It must be Sarah).

When we are almost 100% sure something is not true, we use can't (It can't be the pizza).

Can is not used for specific present deductions. Mustn't is usually used for prohibition (rules against doing something), not for negative deductions.

Question 6

Help the dog owner figure out what her pet is feeling by choosing the correct word.

Look at Buster drooling over that fallen slice of pepperoni pizza. He hasn't eaten all day, so he ___ be absolutely starving!

The correct answer is must.

When we are almost 100% sure that something is true based on present evidence (like drooling and not eating all day), we use the modal verb must for deduction.

"Might" is only used when we are unsure, and "can't" is used when we are sure something is impossible.

Question 7

Choose the correct modal verb for each gap to complete the friends' guesses about the mysterious birthday gift sitting on the table.

It rattles lightly when you shake it. It could be a board game, but I'm not entirely sure.

It is completely flat and bends easily. It can't be a new laptop because laptops are rigid and heavy!

Wait, the box has a "Live Animal" sticker and is meowing loudly. It must be the little kitten you asked for!

It rattles lightly when you shake it. It could be a board game, but I'm not entirely sure.

We use could to express a possibility when we are guessing but lack total certainty.

It is completely flat and bends easily. It can't be a new laptop because laptops are rigid and heavy!

We use can't for a strong negative deduction. The evidence (it bends) makes it impossible to be a laptop.

Wait, the box has a "Live Animal" sticker and is meowing loudly. It must be the little kitten you asked for!

We use must for a strong positive deduction. The clues (sticker and meowing) make it almost certain to be a kitten.

Question 8

Help the amateur office detective complete her notes about the missing chocolate cake by selecting the most logical modal verbs.
"Look at Dave! He has chocolate frosting all over his mouth. He _________________________ be the thief!
But Sarah is severely allergic to chocolate, so she _________________________ be the culprit.
What about Jim? He's looking a little bit nervous in the corner, so he _________________________ know something about it. I'll interrogate him next!"

must

We use must when we are almost entirely sure something is true based on present evidence (Dave has frosting on his face).

can't

We use can't when we are almost entirely sure something is NOT true (Sarah is allergic, so it is impossible that she ate it).

might

We use might (or may/could) when we think something is possible but we aren't completely sure (Jim looks nervous, which is a clue, but not absolute proof).

Question 9

Choose the correct words to complete the confused student's thoughts as he arrives at an empty classroom.

The classroom is completely dark and empty. I must be in the wrong room.

Wait, the schedule says Room 302, which is this room. The class might be on a field trip today, but I don't remember the professor mentioning it.

It is only 9:05 AM, and this is a two-hour lecture, so the lesson can't be over already!

The classroom is completely dark and empty. I must be in the wrong room.

The empty, dark room provides strong evidence for the student to conclude he is in the wrong place. We use must for strong positive deductions.

Wait, the schedule says Room 302, which is this room. The class might be on a field trip today, but I don't remember the professor mentioning it.

The student is guessing a possible reason but isn't sure. We use might to express possibility.

It is only 9:05 AM, and this is a two-hour lecture, so the lesson can't be over already!

Since the class just started 5 minutes ago, it is logically impossible for it to be finished. We use can't for strong negative deductions.

Question 10

Help the cautious diner make a safe choice! Select ALL the options that correctly complete the sentence.
"I'm really not sure what's in this mysterious green soup. It _____ vegetarian, so you probably shouldn't eat it if you strictly avoid meat."

The correct answers are might not be and may not be.

Because the speaker says "I'm really not sure," they are expressing a negative possibility (there is a 50% chance it is NOT vegetarian). We use might not or may not for this.

Can't be and couldn't be would mean the speaker is 99% certain the soup is impossible to be vegetarian (i.e., they know for a fact it has meat in it), which contradicts "I'm really not sure."

Question 11

Help the confused homeowner deduce what is inside a mystery package left on the porch.
"This box is as light as a feather! It _________________________ be the heavy bowling ball I ordered.
Wait, it's shaped exactly like a guitar case, so it _________________________ be that vintage acoustic guitar you won at the auction!
Let me check the label... the address is slightly smudged. It _________________________ belong to our neighbor, Mr. Henderson, but I'm not entirely sure."

can't

A light box cannot possibly contain a heavy bowling ball, so we use can't for a strong negative deduction.

must

The distinct shape of a guitar case provides strong evidence, so we use must for a confident positive deduction.

might

The smudged label leaves room for doubt. The speaker explicitly says "I'm not entirely sure," making might the perfect choice for a possibility.

Question 12

Help Detective Paws complete his investigation notes about the missing cookies by choosing the correct modal verb for each blank.

The cookie jar is empty and Buster the dog has chocolate crumbs on his nose. He must be the culprit!

However, Whiskers the cat is fast asleep in the other room. She can't be the thief because she hasn't moved from her bed all day.

There is an open window in the kitchen, so a sneaky neighborhood squirrel might be involved, but we need more evidence to prove it.

The cookie jar is empty and Buster the dog has chocolate crumbs on his nose. He must be the culprit!

We use must when we are almost 100% sure something is true based on strong present evidence (the crumbs).

However, Whiskers the cat is fast asleep in the other room. She can't be the thief because she hasn't moved from her bed all day.

We use can't when we are almost 100% sure something is NOT true. If she has been asleep all day, it's impossible that she stole the cookies.

There is an open window in the kitchen, so a sneaky neighborhood squirrel might be involved, but we need more evidence to prove it.

We use might (or may/could) when we think something is possible, but we aren't completely sure.

Question 13

Complete the student's shocked text message to their friend with the correct modal verb.

That guy in the mosh pit ___ be Professor Higgins! He is supposed to be giving a lecture in Tokyo right now!

The correct answer is can't.

When we are almost certain that something is impossible or not true, we use can't (or cannot).

Careful! A very common mistake is to use "mustn't" as the opposite of "must" for deduction. However, "mustn't" is only used for prohibition (e.g., "You mustn't walk on the grass"), never for deduction!

Question 14

Join the roommates in their desperate hunt for the missing TV remote by selecting the right word.

I'm not entirely sure where the remote went. It ___ be under the couch cushions, or maybe the cat hid it again.

The correct answer is could.

We use could, might, or may when we are guessing about a present situation but aren't completely sure.

Since the speaker says "I'm not entirely sure" and offers another option ("or maybe the cat hid it"), "must" (certainty) and "can't" (impossibility) are too strong here.

Question 15

Complete the office worker's guess about the mysterious phone call.

The caller ID is blocked. It ___ be the delivery driver with our pizza, but they usually call from a local number. Let's answer it just in case.

The correct answer is might.

The speaker thinks it is possible that the caller is the delivery driver, but the word "but" introduces doubt. Because there is uncertainty, might is the perfect choice.

"Must" would mean they are absolutely sure, while "can't" would mean they think it's completely impossible.

Terms of ServicePrivacy Policy
Copyright Kind Robots Ltd.