PRESENT SPECULATION
You see that your neighbor's lights are out, the driveway is empty, and there is a padlock on the gate. You are 100% sure they are not home.
"They __________ be at home. Everything is locked up and the car is gone."
can't
You wake up and see that the streets are completely wet and there are huge puddles everywhere. You are certain it rained during the night.
"It __________ rained last night; the ground is completely soaked."
must have
You are standing inside a hospital where smoking is strictly forbidden by law.
"You __________ smoke inside the hospital. It's strictly prohibited."
mustn't
It is 40°C outside. You give someone a strong warning not to go out without sunscreen so they don't get a sunburn.
"It's scorching outside. You __________ better not go out without sunscreen."
had
Yesterday was a public holiday, so there was no school. Because I knew this in advance, I stayed in bed and didn't go.
"Yesterday was a holiday, so I __________ go to school."
didn't need to
Someone rings the doorbell. You ordered a pizza, but it's 20 minutes earlier than expected, so you aren't completely certain it's the delivery guy.
"That __________ be the pizza delivery, but it's a bit too early."
might / could / may
You cannot find your keys anywhere. You think it's possible you left them on your desk at work, but you aren't 100% sure.
"I can't find my keys. I __________ left them at the office, but I'll check my bag again."
might have / could have / may have
Tomorrow is Sunday and your school/office is closed. You have absolutely no obligation to get out of bed early.
"Tomorrow is a holiday, so I __________ wake up early."
don't have to or needn't
If we don't leave the house right this second, we will definitely miss our flight. This is an urgent warning with a bad consequence.
"We __________ leave right now, or we will miss our flight!"
had better
You bought two loaves of bread on your way home. When you walked into the kitchen, you saw that your mom had already bought four loaves. You wasted your money.
"I __________ all this bread. We already have plenty in the kitchen!"
needn't have bought
Anna owns three luxury sports cars, a massive mansion, and travels to exotic islands every month. You are certain she has a lot of money.
"She __________ have a lot of money to afford that kind of lifestyle."
must
John was in London with you all day yesterday. Someone claims they saw him sitting in a café in Paris at the exact same time. It is impossible.
"You __________ seen John in Paris yesterday. He was in a meeting with me in London the whole day."
can't have (or couldn't have)
Company rules state that all employees are required to wear their ID badges. This is an external rule imposed by management.
"According to company policy, we __________ wear our ID badges at all times."
have to
You are telling someone they need to behave better towards their parents because it is the right thing to do.
"You __________ to treat your parents with more respect."
ought
You spent five hours cleaning your apartment before your friends arrived. When they got there, they said, "We didn't care about the mess at all!"
"I __________ the whole house. They said they really didn't mind a bit of dust."
needn't have cleaned
Mark is not answering his phone. You know he is either in an important meeting or his phone battery died. There are multiple possibilities.
"He isn't replying. His phone __________ be dead, or he is in a meeting."
might / could/may
The teacher was furious when she entered the classroom and looked at everyone's empty desks. She assumes the students forgot their homework.
"The teacher was furious. The students __________ forgotten to do their homework."
must have
You see a security sign at the airport that says "Authorized Personnel Only".
mustn't
Your friend has been coughing severely for days. You want to give them some standard, friendly advice to see a doctor.
"You look terrible. You __________ see a doctor about that cough."
should / ought to
The hotel website stated that towels are provided for free in every room. Therefore, it wasn't necessary to pack them.
"The hotel provided everything, so we __________ pack any towels in our bags."
didn't need to
You hear a horrific, loud grinding noise coming from the engine of a brand-new car. You are absolutely certain something is wrong, even if you aren't a mechanic.
"Listen to that terrible noise! There __________ be a serious problem with the engine."
must
A thief managed to get inside a house on the second floor. Later, the police noticed that a second-floor window had been left unlocked. It was one of the few possible entry points.
"The window was unlocked. The thief __________ climbed through it to get inside."
could have / might have
The museum is completely free for university students, meaning they are under no obligation to buy a ticket.
"Admission is free for students, so you __________ pay for a ticket."
don't have to/or need not / needn't
They failed their exam because they played video games instead of studying.
"You failed the test. You __________ studied much harder last week."
should have / ought to have
"Because the event was cancelled early, we didn't need to pay a fee. However, we sent them an expensive apology gift anyway, which wasn't necessary at all."
"We sent them a luxury gift as an apology, but we __________ it, as the cancellation wasn't our fault."
needn't have done