[Grammar] How to use "Not only...,but also..." with "could"

I have a question about using "could" in the sentence which has "Not only...but also..."

The following sentence is incorrect, but it sounds grammatically correct to me.
Not only it could reduce waste, but also it would produce energy.

I had this sentence corrected by a native speaker, and he said "could" in this sentence should be put after the subject as follows.

Not only could it reduce waste, but also it would produce energy.

I am wondering if how this grammar rule be applied in other sentence, or it just an exceptional that I have to remember.

Thanks!!!

When "not only" starts the sentence, the subject and verb that follow are reversed.

Barb_D said:

When "not only" starts the sentence, the subject and verb that follow are reversed.

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Yep. That is absolutely correct and it seems like one of those weird rules which are just true because they are "true" but there's no logic to it.

Gabbi

The rule is not that random. Whenever you have a negative phrase start your sentence, you invert.

Scarcely had I started when...
Never will you find...
Rarely had he seen...
Not only is this...

Barb_D said:

The rule is not that random. Whenever you have a negative phrase start your sentence, you invert.

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Hmmm. You are right. How weird. True. But weird. Funny thing is I don't ever recall anyone actually pointing that out before (or I wasn't listening).

Gotta love English.