By this level you've already met most of these individually — must/might/can't have for guessing about the past (deduction), and should/could/would have for regret, criticism and missed chances. This page doesn't reteach them from scratch — it puts every form side by side so you can tell them apart at a glance, with mixed practice at the end.

Deduction: sure vs. unsure Regret & criticism Full comparison table Exercises

Deduction about the past: how sure are you?

These guess whether something happened — they don't judge if it was right or wrong.

FormCertaintyExample
must have + ppAlmost certain it happenedHe must have left already — his coat is gone.
might have / may have + ppPossible, not sure either wayThey might have missed the train.
could have + ppPossible (a real option existed)It could have been an accident.
can't have / couldn't have + ppAlmost certain it did NOT happenShe can't have finished — it's only been five minutes.
Full deep dive: These forms are covered in detail, including deduction about the present too, in the B1 Modals of Probability page.

Regret, criticism & missed chances

These judge a past action — was it the right thing to do, and did it actually happen?

FormMeaningExample
should have + ppRegret/criticism — the right thing wasn't doneYou should have called me! (but you didn't)
shouldn't have + ppRegret/criticism — the wrong thing was doneI shouldn't have eaten so much. (but I did)
could have + ppMissed opportunity / mild criticismYou could have told me! (it was possible, but you didn't)
would have + ppUnfulfilled plan, or 3rd conditional resultI would have come if I had known.
needn't have + ppDid it, but it was unnecessaryYou needn't have cooked — we already ate.
Full deep dive: These forms — including "needn't have" vs. "didn't need to" — are covered in full on the Could / Would / Should Have page.

The one question that tells them apart

Ask yourself: Am I guessing whether something happened (deduction — must/might/could/can't have), or am I judging whether it was the right thing to do (regret/criticism — should/shouldn't/could/would/needn't have)? "Could have" is the one form that appears in both columns — context tells you which sense is meant.
"He must have forgotten the meeting." (deduction — a guess)
"He shouldn't have forgotten the meeting." (criticism — he's at fault)

Exercises

Choose the past modal that fits the meaning in brackets.

1. The lights are off and the car's gone. They ___ already. (deduction)

2. You knew the roads were icy. You ___ so fast! (criticism)

3. It's impossible — she ___ the exam already, the results aren't out yet. (impossibility)

4. We already had food at home — you ___ more! (unnecessary action)

5. I'm not sure, but he ___ the email — check with him. (uncertain possibility)

6. If I had known you were free, I ___ you. (unfulfilled plan)

← Could / Would / Should Have Passive with Modal Verbs →