Read about using modal verbs for deduction (saying what you think is true). Then, when you are ready, try these modal verbs practice exercises.
Must, might and can't for deduction
Use must if you are very sure about something:
Look at her teeth! She must be a vampire.
Use might (or could or may) if you think something is possibly true.
Be careful. That vampire might be hungry.
Use can't if you are sure something is impossible.
She can't be a vampire - they don't exist!
Note: We never use can or mustn't with this meaning - only must and can't.
How to use modal verbs for deduction
Modal verbs are always followed by the infinitive (without 'to'). But you need to decide if you are talking about a state, an action in progress, or a finished action/state.
He might be at home now. (modal + infinitive)
Actions in progress now
He might be working now. (modal + be + verb-ing)
He might have gone out. (modal + have + past participle)
Use the same patterns for must (must do / must be doing / must have done) and can't (can't do / can't be doing / can't have done).
If you can't remember the difference between actions and states, you can read more about it on my state verbs page.
Now test yourself with these modal verbs practice exercises.