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I know from verious other discussions that 'can not' and 'cannot' are basically both correct but 'cannot' is being used more often. Nonetheless what happens if I have a 'not only...but also' construction like in the following sentence:

"Heat pumps cannot only be used for space heating but also for space cooling."

Is it also correct to use 'cannot' in this context? I'd appreciate every comment.

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    I'd prefer to say not only can heat pumps be used for space heating, but also for space cooling. Commented Apr 18, 2020 at 15:50
  • Why not use "either,,,,,, or"
    – Centaurus
    Commented Apr 18, 2020 at 15:53
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    'Not only... but also...' structure is inversion. If you want to invert your sentence, you need to split up 'cannot' and put 'not' at the beginning. Commented Apr 18, 2020 at 16:00
  • As written, it is a denial that heat pumps can only be used for space heating. It communicates differently from the “can [not only ... but also]” variant.
    – Lawrence
    Commented Apr 18, 2020 at 16:02
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    I thought I included all the relevant text in my actual comment, so you shouldn't have even needed to follow the link. Yes - you should split cannot in your context, because not forms part of the collocation not only (itself part of the standard not only X but also Y construction), rather than negating the verb can. Commented Apr 18, 2020 at 18:18

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You have two problems here. Let's start with correlative conjunctions and parallelism . . .

Correlative conjunctions—either/or, neither/nor, not only/but [also]—come in pairs. What follows each part of the pair must be parallel: both nouns, both verbs (same tense), both prepositional phrases, both independent clauses. In the case of your sentence, it should read:

Heat pumps can be used not only for space heating but also for space cooling.

or

Heat pumps can be used for not only space heating but also space cooling.

Now you have parallel prepositional phrases or parallel noun phrases. And, voila, the use of cannot is impossible.

But even if we used parallel verbs, cannot would still be incorrect:

*Heat pumps cannot only be used for space heating but also be used for space cooling. (incorrect)

You need to use can not. Why? Simply because not belongs to the correlative conjunction pair not only/but also—not to can:

Heat pumps can not only be used for space heating but also be used for space cooling.

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  • Thanks Tinfoil Hat for your answers. Is the last sentence your final adivce:"Heat pumps can not only be used for space heating but also be used for space cooling."
    – PeterBe
    Commented Apr 18, 2020 at 17:22
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    The first or second would be my preference. The first sounds a little better to me, even though using for twice makes it wordier. Commented Apr 18, 2020 at 17:50
  • Thanks for the answer Tinfoil. Can I basically also use the constrction:"can not only... but also" instead of "not only can ... but also". For me this sound better?
    – PeterBe
    Commented Apr 18, 2020 at 20:06
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    @PeterBe: You can do that, but the sooner you introduce not only, the more you have to repeat in order to maintain the parallel structure: Heat pumps not only can be used for space heating but also can be used for space cooling. Also, if you do want to branch at the verb, it is more natural to do it after the auxiliary (in this case, can), which would be the last example in my answer. Again, the most natural is the first sentence. Commented Apr 19, 2020 at 0:58
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    Ah, then this is the perfect case for the dummy there: There are meters currently being rolled out that can not only monitor electricity generation and demand at high temporal resolution but also communicate with other components in the grid. Commented Apr 19, 2020 at 15:27

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