Modal + perfect infinitive (might have been, Murphy Unit 43)
Pattern
Section titled “Pattern”Modal + have + V3 — the modal’s “past tense”. Applies to ALL modals, each with its own stance toward a past event.
| Modal + have + V3 | Meaning | Example |
|---|---|---|
| might have | past possibility (unsure) | It might have been caused by a fault. |
| may have | past possibility (formal) | She may have left early. |
| could have | past possibility OR unrealized ability | He could have won. (was possible / didn’t) |
| must have | past logical deduction (sure) | You must have been tired. |
| can’t have | past negative deduction (ruled out) | He can’t have done it. |
| should have | past obligation not met (regret/criticism) | You should have told me. |
| ought to have | same as should have | I ought to have called. |
| would have | past hypothetical (if-result) | I would have come. (but I didn’t) |
| needn’t have | past unnecessary action | You needn’t have bothered. (but you did) |
Why this is NOT past perfect
Section titled “Why this is NOT past perfect”might have been caused looks like past perfect (had been caused) but the engine is different:
| Form | Job | |
|---|---|---|
| Past perfect | had + V3 | unreal past (hypothetical backshift in if/wish) |
| Modal + perfect infinitive | modal + have + V3 | speaker’s stance toward a past event (possibility, deduction, regret) |
| Present perfect | has + V3 | real, up to now |
The have after a modal is the perfect infinitive (to have done), not the past-perfect auxiliary.
Distinguishing the three “have been + V3” shapes
Section titled “Distinguishing the three “have been + V3” shapes”| Shape | Meaning | Example |
|---|---|---|
| might have been caused | past possibility (unsure) | It might have been caused by a fault. |
| had been caused | unreal past (hypothetical/regret) | If it had been caused by a fault… |
| has been caused | present perfect (real, up to now) | It has been caused by… (still true now) |
Same verb shape, different engine. The “unreal/hypothetical” reading only appears when context (if/wish) demands it; modal + have is stance, not hypothetical.
The must / must have / had to split (obligation vs deduction)
Section titled “The must / must have / had to split (obligation vs deduction)”must has two meanings — obligation (harus) and deduction (pasti). The past forms are completely different:
| Time | must = OBLIGATION | must = DEDUCTION |
|---|---|---|
| Present | You must wear a helmet. (required) | You must be tired. (I deduce) |
| Past | had to — I had to work yesterday. (was required) | must have + V3 — You must have been tired. (I deduce) |
Key rule: the past of obligation is had to, NOT must have. Must have NEVER means “was obliged to” — it always means “surely did”:
- ✗ I must have work yesterday (obligation) → ✓ I had to work yesterday
- ✓ The door is locked — he must have left. (deduction)
Why “must have” = deduction (the user’s insight): must have = past obligation assumed fulfilled → the speaker reasons backward from present evidence: “given the obligation existed and nothing prevented it, the event certainly happened.” The fulfilled-obligation reading IS the deduction.
Should vs must for criticism/advice:
| should | must | |
|---|---|---|
| Strength | advice / recommendation (soft) | strong obligation (hard) |
| Past criticism | should have + V3 — You should have waited. (you failed) | no past form for criticism |
| Past deduction | — | must have + V3 — You must have waited. (I deduce) |
- You should have waited = criticism (you didn’t wait — the wrong you did)
- You must have waited = deduction (I’m sure you waited) — wrong for criticism, because the speaker KNOWS they didn’t wait (“why did you leave without me?”)
The one overlap: must as a STRONG should (recommendation near-obligatory):
- You really must try the pasta = you really should (emphasis)
- We must do something soon = urgency
Cure-Dolly: must have = the modal looking BACKWARD from the present — “given present evidence, the past event is certain.” The obligation sense in the past is a different auxiliary entirely: had to.
Test application
Section titled “Test application”It’s not certain how the fire started. It might ___ (cause) by an electrical fault.
Answer: have been caused — modal (might) + perfect infinitive (have been) + V3 (caused), passive.
- might be caused = present possibility (wrong — fire already started)
- might have been caused ✓ = past possibility, passive
- might was caused ✗ — modals never take past tense directly
Source
Section titled “Source”Raymond Murphy, English Grammar in Use (4th ed.), Unit 43 — Passive 2 (be/been/being done). Ebot lesson 2026-08-29: “It might ___ (cause) by an electrical fault” → have been caused. User insight: “modal + perfect infinitive is the true general form” — yes, applies to all modals.