Why is that man looking around like that? He must be lost.
(We use "must" because it seems like a logical conclusion.)

That woman can't be a doctor! She looks far too young.
(We use "can't" because it suggests disbelief or impossibility based on her appearance.)

John always fails the tests, even though he's clever. He mustn't study enough.
(This suggests that the reason for his failure is probably because he doesn’t study enough.)

The food is really good at that restaurant. They must have a great chef.
(We use "must" to indicate a strong possibility based on the high quality of the food.)

Who's that at the door? It can't be Susie - she'll still be at work now.
(We use "can't" to indicate that it's impossible for it to be Susie at this time.)

This must be John's house. This house has a red door and it's number 24, just like he said.
(We use "must" because the evidence strongly suggests that it is John's house.)

Julie can't have much money or she would buy a new car. Her old one is falling apart.
(We use "can't" to indicate that it's impossible for her to have much money, as she hasn't bought a new car.)

He can't be at work now, can he? It's nearly midnight.
(We use "can't" because it is highly unlikely he would be at work at such a late hour.)

What a lot of lovely flowers you have! You must really like gardening.
(We use "must" to show a strong assumption based on the evidence - the flowers.)

David must drink a lot of coffee. He's finished two packets already this week.
(We use "must" to show a logical conclusion based on the amount of coffee he has consumed.)

This can't be Jamie's coat. He's tall and this is tiny.
(We use "can't" because it is impossible for this coat to be Jamie's due to its size.)

Her life can't be easy. She has four children and very little money.
(We use "can't" because it seems very unlikely that her life is easy given her circumstances.)

Where's Lucy? She must be at the library, as she often goes there at this time.
(We use "must" because it's a logical assumption based on her regular behavior.)

This bill can't be right! £50 for two cups of coffee!
(We use "can't" to express disbelief and that it's impossible for the bill to be correct.)

Emma's amazingly good at the piano. She must practise a lot.
(We use "must" to indicate a strong conclusion based on her skills.)

The car in front is driving so slowly that I think they must be looking for something.
(We use "must" because it’s a logical assumption based on the slow driving.)

You've already eaten enough for three people! You can't still be hungry!
(We use "can't" to show disbelief and to suggest that it is impossible for them to be hungry.)

This book can't belong to the library. It's certainly not mine.
(We use "can't" because it is impossible for the book to belong to the library.)

It only takes three hours to fly from London to Sydney? That can't be correct!
(We use "can't" to show disbelief or doubt about the information.)

There must be something wrong with the fridge! It's making a very unusual noise.
(We use "must" to show a strong assumption based on the unusual noise.)

Why is that man looking around like that? He must be lost.

That woman can't be a doctor! She looks far too young.

John always fails the tests, even though he's clever. He mustn't study enough.

The food is really good at that restaurant. They must have a great chef.

Who's that at the door? It can't be Susie - she'll still be at work now.

This must be John's house. This house has a red door and it's number 24, just like he said.

Julie can't have much money or she would buy a new car. Her old one is falling apart.

He can't be at work now, can he? It's nearly midnight.

What a lot of lovely flowers you have! You must really like gardening.

David must drink a lot of coffee. He's finished two packets already this week.

This can't be Jamie's coat. He's tall and this is tiny.

Her life mustn't be easy. She has four children and very little money.

Where's Lucy? She must be at the library, as she often goes there at this time.

This bill can't be right! £50 for two cups of coffee!.

Emma's amazingly good at the piano. She must practise a lot.

The car in front is driving so slowly that I think they must be looking for something.

You've already eaten enough for three people! You can't still be hungry!.

This book can't belong to the library. It's certainly not mine.

It only takes three hours to fly from London to Sydney? That can't be correct!

There must be something wrong with the fridge! It's making a very unusual noise.