Element FORM/USE: 'IF' + PAST PERFECT + MODAL VERBS IMAGINED SITUATION, REGRET
SuperCat CLAUSES
SubCat conditional
Lexical Range N/A
Level C1
Cando Can form conditional subordinate clauses with 'if' + the past perfect simple and modal verb + 'have' + '-ed' in the main clause, to talk about imagined situations in the past, often with regret.
Corrected Learner Example All this could have been avoided, if the plan had been more flexible. (Poland; C1 EFFECTIVE OPERATIONAL PROFICIENCY; 1993; Polish; Pass)

Several statements in the newspaper could have been avoided, if the writer had taken notice of the income results which were sent along with the other facts from the charity day. (Denmark; C1 EFFECTIVE OPERATIONAL PROFICIENCY; 2000; Danish; Pass)

However, I think I could have learned more if more individual tuition had been available. (Poland; C1 EFFECTIVE OPERATIONAL PROFICIENCY; 1993; Polish; Pass)

I wrote this article referring to my own experience in London, hence I might have had a different opinion if I had gone to another city or country. (Italy; C1 EFFECTIVE OPERATIONAL PROFICIENCY; 1999; Italian; Pass)
Uncorrected Learner Example
Comments 'could have' + -ed is the most frequent form in the main clause after 'would have' + -ed. There are very few examples with 'might have' + -ed.
Note on punctuation: many grammar books stress that when the if clause precedes the main clause, it must use a comma. We cannot establish a consistent competency for this (and punctuation in general). It seems too idiosyncratic. L1 can be a skewing factor.

Cambridge University Press