What you'll learn
By the end of this lesson, you'll be able to express how certain you are about something using must, might, could and can't.
When do I use it?
Use modal verbs to make logical guesses about the present:
- must — you are almost sure it is true: "He's not answering. He must be asleep."
- might / may / could — it is possible: "She might be at home." "The keys could be in the car."
- can't — you are almost sure it is false: "That can't be John, he's on holiday."
Notice the scale: can't (impossible) → might/may/could (possible) → must (almost certain).
How do I form it?
subject + must / might / could / can't + base verb
- You must be tired after that long flight.
- It might rain later, take an umbrella.
- The noise could come from next door.
- He can't be hungry, he just ate.
The modal is followed by the base verb — no "to", no -s, no -ing.
Common mistakes for Spanish speakers
Spanish speakers often use "should" for probability ("debe de ser verdad") and translate it as "should be true" — that is wrong. "Debe de ser" is "must be", and "no puede ser" is "can't be", not "don't can be" (modals take no auxiliary in the negative). Also, do not add "to" after the modal ("must to go") or use the wrong infinitive form ("must be tired" vs "must to be tired").
Almost sure yes → must. Possible → might / may / could. Almost sure no → can't.
Practice
Multiple Choice
B1Which sentence is correct for 'almost sure it's false'?
Fill in the Blank
B1You've driven for six hours. You be exhausted.
Hint: Almost certain deduction → must + verb.
Find the Error
B1Multiple Choice
B1Which sentence means 'it is possible'?
Practice Speaking
Apply what you've learned in a real conversation with AI feedback.
Level: B1