When do I use 'must be' instead of 'might be'?

Use 'must be' when the evidence makes you almost certain something is true, as in "He must be at work." Use 'might be', 'may be', or 'could be' when it's only one possibility among several, as in "She might be on her way."

Why is 'He mustn't be home' wrong?

'Mustn't' means something is forbidden, not that it's impossible. For a negative deduction — when you're sure something isn't true — the opposite of 'must be' is 'can't be'. So you should say "He can't be home," not "He mustn't be home."

Are 'might be', 'may be', and 'could be' the same?

Yes, for present deductions they all express possibility and are interchangeable in most everyday situations. "She might be on her way," "She may be on her way," and "She could be on her way" all mean roughly the same thing.

Verbs

Modals of Deduction: must be, can't be, might be

Level B1 Verbs

Key idea

Modals of deduction let you say how certain you are about something happening right now. Use 'must be' when the evidence makes you sure it's true ("He must be at work"), and 'can't be' when you're sure it's false ("That can't be true"). For something that's only possible, reach for 'might be', 'may be', or 'could be' ("She might be on her way"). The key trap: the opposite of 'must be' is 'can't be', not 'mustn't be' — so never say "He mustn't be home."

Examples

He must be at work. the speaker is sure he is at work
That can't be true. the speaker is sure it is false
She might be on her way. it is possible she is coming

The full lesson

Everything in the video, in text.

must be · can't be · might be

how sure are you?

The phone rings out. He must be busy. English lets you turn evidence into a guess — and there's one trap almost everyone falls into.
Modals of deduction = how sure you are about the present.

These are modals of deduction. You're not stating a fact — you're reading the evidence and saying how certain you are it's true, right now.
The certainty scale
Sure it's TRUE
must be
Sure it's FALSE
can't be
Picture a scale. At the top, you're almost certain it's true: must be. At the bottom, certain it's false: can't be. In the middle, just possible: might, may, or could be.
He must be at work.
near-certain · positive

Start with strong certainty. The lights are on, his car's outside — you're sure. He must be at work.
That can't be true.
near-certain · negative

Now the opposite. The evidence rules it out completely — so you reach for can't be. That can't be true.
She might be on her way.
possibility

And when you simply don't know, drop into the middle. Might, may, or could — all just possible. She might be on her way.
She must be asleep.
a conclusion from evidence

It works beautifully with evidence in front of you. She's not answering her phone, so you conclude: She must be asleep.
✗ He mustn't be home. ✗ not used for deduction

✓ He can't be home. I'm sure he isn't home

The opposite of 'must be' is 'can't be' — never 'mustn't be'.

Here's the trap. The opposite of must be is NOT mustn't be. To say you're sure something is false, English uses can't be. He mustn't be home sounds wrong to native ears.
He can't be home.
the correct negative

Compare them directly. You knocked and waited — nothing. So you're sure he isn't there. He can't be home.
Modal + be → must be, can't be, might be.

One more thing: deduction is about now. After the modal, use the base verb be — must be, can't be, might be. Don't add an extra is.
✗ She must is tired. ✗ double verb

✓ She must be tired. modal + be

Modal + base verb. No extra 'is'.

So it's never must is or can't is. The modal already carries the tense — just add be.
Remember
- must be → sure it's TRUE
- can't be → sure it's FALSE
- might / may / could be → possible
So: must be for sure-it's-true, can't be for sure-it's-false, might be for maybe. Read the evidence, pick your level.

Examples

must be · can't be · might be

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