Where it appears in the exam
What is it?
Possibility modals (might, could, may) express that something is possible but not certain. In English, certainty is expressed through a SPECTRUM of modals: can't (impossible) to might (possible) to should (likely) to must (almost certain). For the past, add 'have + past participle': might have, could have, must have. Cambridge tests that you choose the right point on the scale.
Why it matters in the exam
This is the most tested modal group in Part 1 (MCQ with 4 modals at different certainty levels) and Part 2 (produce 'might/must/can't' + have for past speculation). It also appears in Part 4 with 'perhaps' to 'might' transformations.
The cognitive trap
"Maybe it will rain. / It might rain. / It could rain."
Your brain reaches for adverbs -- 'maybe', 'perhaps', 'possibly' -- instead of modal structures. Adverbs feel safer because they don't require you to calibrate the certainty level through grammar.
"It might rain" is not "It should rain" is not "It must be raining"
In English, each modal indicates a different certainty level. Cambridge demands the exact modal for the context. Adverb-based workarounds ('Maybe it rains') don't demonstrate the competency being tested.
Recognition pattern
In the exam, look for the key signal first. The answer follows.
Certainty level signals
"She might be at the office."
"He should arrive before noon."
"He must be tired -- he's been working all day."
"That can't be right."
"They might have forgotten about the meeting."
"You could have warned me!"
"She might not come" vs "She can't be serious"
"Perhaps he is lost" becomes "He might be lost"
Structure and certainty errors
"Maybe he is stuck in traffic."
Grammatically correct, but in Part 2 Cambridge tests the MODAL STRUCTURE, not the adverb. 'Maybe' + present simple is not what's being tested.
"He might be stuck in traffic."
Might + infinitive for present speculation. Correct modal structure.
"She might missed the train."
Missing 'have'. The past structure is ALWAYS modal + HAVE + past participle. Never modal + past simple.
"She might have missed the train."
Might + have + PP for past speculation. Complete structure.
"You could tell me earlier!"
Without 'have' = present ability/possibility. With 'have' = past unrealised. The meaning changes completely.
"You could have told me earlier!"
Could have + PP = unrealised possibility. Reproach: you could have told me but you didn't.
Why your brain gets it wrong
The learner's short circuit
Analyse the trap by exam format
I haven't heard from Tom today. He 4 be ill — he wasn't feeling well yesterday.
Your brain picks 'must' (he must be ill). But 'must' = 95% certain. Here there's only a weak clue ('wasn't feeling well'). You're NOT SURE -- just speculating. Might = possibility without certainty.
Must and might are not interchangeable
Must = I'm almost certain, might = it's just possible. The instinct to jump to 'must' comes from overgeneralizing deduction -- your brain wants closure, not uncertainty.
Strategy
Evaluate the speaker's CERTAINTY: is there strong, direct evidence? Then must. Is there only a weak clue or speculation? Then might/could.
Nobody answered the phone. They might ______ gone out for the evening.
Your brain writes 'has' by subject-verb agreement with 'they' -- but that's wrong. After a modal, it is ALWAYS 'have' (infinitive), never 'has'. Modals don't conjugate.
might _____ gone
Modal + HAVE (always infinitive) + PP. 'Might have gone', 'could have left', 'must have forgotten'. Never 'has/had'.
→ have
After a modal, always HAVE (never has/had)
Modals (might, could, must, should) don't conjugate. The verb that follows always goes in the infinitive: 'might have', not 'might has'. Subtle but frequent error.
Strategy
Absolute rule: modal + HAVE + past participle. For any subject: I/you/he/she/they might HAVE gone.
Perhaps the letter got lost in the post. (MIGHT) The letter ______ in the post.
'Got lost' is in the past, so you need the past modal structure: might + HAVE + past participle. Without 'have', it sounds like future speculation ('maybe it will get lost').
MIGHT
'Perhaps' + past = might have + PP. 'Perhaps' + present = might + infinitive.
→ might have got lost
The tense of the original sentence determines the structure
If the original sentence is in the past ('got lost'), the modal transformation is also past: might HAVE got lost. If it's in the present, it's might + infinitive.
Strategy
Check the tense of the original sentence. Past = modal + have + PP. Present = modal + infinitive.
Possibility & Probability is 1 of 82
The exam tests 82 grammar competencies across 19 families. Mastering one is the first step. Automating all 82 is passing.
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