If I were her, I would have killed him months ago. [englishforum]
A person asks whether the expression can be acceptable. In my mother tongue, that also can be a way of saying. In English, is it a proper saying?
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This is indeed exactly the right way to construct an IF...THEN expression with a counterfactual (a ‘condition contrary to fact’) in the IF clause:
For your IF clause, you employ the ‘past subjunctive’ (or whatever your particular grammatical sect chooses to call it); this is the basic past form of your verb, without personal inflection — in this case, were.
For your THEN clause, you start with the past form of a modal verb to mark the act as hypothetical: would ...
... together with the ‘bare infinitive’ of your lexical verb, which would be kill, except that ...
... in your case you want to say not that you would kill in the present or future, but that this hypothetical act of violence would by now be an already accomplished fact. Accordingly, you backshift the infinitive kill into the past by employing the bare perfect infinitive: have + the past participle of your lexical verb, killed.
The answer at englishforums.com suggests that forms like “If I were him, I would do...” and “If I had been him, I would have done...” are legitimate, and that form “If I were him, I would have done...”, while commonly heard, may be questionable.
That might be technically correct – I can't quite say – and in reviewing the subjunctive in English I was unable to tell for certain. Here in the USA I hear a vast variety of subjunctive forms and the form “If I were him, I would have done...” seems to me clear and understandable rather than wrong.
Digression: The problem I have with “If I had been her, I would have killed him months ago” or “If I had been him, I would have done ...” is that they seem to pile a second counter-factual on top of the first counter-factual: they suggest that for a while I had been her or him (which is the first counter-factual), and in that while did some action, and after that (when the had been phase ended) changed back to myself. One or the other of those changes does not seem entirely natural to me.