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I was doing some literature homework today, when I became confused as to whether or not the following sentence is grammatical:

X and Y should have killed Z to have prevented Z from killing them.

Should "to" be swapped out for "in order to", or is the whole sentence just plain wrong?

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I think it isn't quite right. X and Y should have killed Z to prevent Z from killing them would work? Possibly adding a time frame if it isn't clear from context (I mean, to emphasize that Z should have killed X and Y in the past) –  Kyudos Feb 15 '13 at 3:04
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3 Answers 3

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The sentence as it stands calls for a simple infinitive, not a perfect infinitive:

X and Y should have killed Z to prevent Z from killing them.

For a perfect infinitive to be appropriate, it would have to be something on the order of:

X and Y would have had to have killed Z to have prevented Z from killing them.

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Is "would" required? –  Chris Irving Feb 15 '13 at 5:44
    
@ChrisIrving Bill Franke and I are arguing about this ... –  StoneyB Feb 15 '13 at 12:16
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It would mean the same thing. In order to can usually be reduced to just to, and in order not to can often be substituted with to or in order to, followed by an antonym for the subsequent verb.

As for whether the whole thing is "just plain wrong" see other posts regarding verb tenses.

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There's nothing at all incorrect in the example sentence X and Y should have killed Z to have prevented Z from killing them. It simply means that X and Y are now dead because Z killed them. Had they killed Z, he would not have been able to kill them and they would not now be dead. If that's not the case, then the sentence is semantically incorrect but grammatically correct.

I disagree with StoneyB about this, but his substitute sentence is perfectly fine, just one of those unreal conditionals that can also be expressed as "If X and Y had killed Z [but they didn't = unreal past condition], that would have prevented Z from killing X and Y". The sentence you ask about is not a conditional but a judgment about what should have been done to prevent the killing of X and Y.

There's no difference between saying "You should have listened to my advice not to buy Apple stock to have prevented (your) losing $10,000 overnight" and "You would have had to have listened to my advice not to buy Apple stock to have prevented (your) losing $10,000 overnight".

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This is tricky; but the subordinate perfect infinitive locates its action as anterior to the event in the main clause; and prevent is semantically incompatible with that. Shift it forward: ✲ I should kill you, to have prevented you killing me doesn't work. –  StoneyB Feb 15 '13 at 10:14
    
@StoneyB: The original is should have killed to have prevented, not should kill to have prevented. The sequence of tenses is fine. Am I missing something. Is one of us having a senior moment here? :-) –  user21497 Feb 15 '13 at 11:38
    
I'm forward-shifting to illustrate the syntax. The infinitive is non-tensed. You can only take an action to prevent something happening in the future, relative to the action; but to have prevented implies something that happened before the action of the main verb, whatever tense that main verb is in. –  StoneyB Feb 15 '13 at 11:46
    
@StoneyB: I said in my answer that the OP's sentence means that X & Y are now dead because Z killed them. They should have killed Z to have prevented him from killing them. Too late now, of course. The only thing we're disagreeing on is your analysis of the grammar of the OP's sentence. I think it's fine. You think it's not fine. I'll wait for John Lawler to weigh in on this point. –  user21497 Feb 15 '13 at 12:00
    
@John Lawler apparently did try to edit my post, but I don't know to what effect: the system rejected his edit as too short. Grumble. –  StoneyB Feb 15 '13 at 12:13
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