modal verbs - tense combined present with past

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?

Answer

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:

1. 
   

   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.

   
   


2. 
   

   For your THEN clause, you start with the past form of a modal verb to mark the act as hypothetical: would ...

   
   


3. 
   

   ... together with the ‘bare infinitive’ of your lexical verb, which would be kill, except that ...

   
   


4. 
   
   

   ... 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.