nonameee wrote:
(1) Headache is not caused by reading or listening, but is by sitting. (here x,y, and z are some nouns).
(2) Headache is caused not by reading or listening, but by sitting.


(1) is incorrect because of 'is' after 'but'.
(2) is correct

IMO, in the first sentence we have a correct parallelism:

NOT verb by noun or noun, BUT verb by noun

The left and the right parts are parallel in that they both have similar grammatical elements.

Could you please explain why 'caused by' cannot be made parallel to 'is by'?

parallelism should be for main verbs

caused is a main verb
is, are, were, was, am, have been, has, had been are all auxiliary verbs/helping verbs.

Sometimes auxiliary verbs can act as main verbs in some sentences as in i am sick, i am XYZ.

Parallelism is tested with main verbs. Don't get confused with main verbs and auxiliary verbs.

Parallelism is tested with prepositions as well. not of ... but of ... not only of ... but also of ...

Hope this helps.

Choice 1 will correct if following the structure "not caused by... but caused by"

or like choice 2 "caused not by... but by"

or "caused by not... but" (although, I do not like this final choice because its sound)

Hope that help

Great explanations from Sam and Tuangquang. Let me just add that the construction "not X but Y" has a fairly straightforward rule to it. What follows "BUT" must be able to stand alone with what precedes "not".

So here if you have:

A headache is not______________________________

The "but ___________" part has to be able to directly follow "is". So a correct answer would be:

A headache is...caused by sitting (too close to the TV?)

An incorrect answer would be:

A headache is...by sitting.

Because a headache isn't an object and sitting isn't a place or object. You could say "my umbrella is by the door", but you couldn't say "my idea is by swimming".

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Think of it this way for structure - the word "NOT" is where the two paths diverge, and each path needs to be directly connected to the "shared" entry. So...

The goal was scored not by Drogba, but by Donovan.

Is correct, because "the goal was scored ... (stop at "NOT" and start after "BUT") by Donovan" is a complete statement.

But

The goal was scored not by Drogba but Donovan.

Is wrong, because if you stop after "NOT" and start after "BUT" you don't have the connector "by":

The goal was scored...Donovan.

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I'd highly recommend looking at the logic of what's written in that way and not trying to just compare "not verb....but verb". As Sam mentioned, there are plenty of different forms of verbs, and a lot of times people get so stuck in comparing grammatical terms that they lose sight of what's really important - logical construction and logical meaning.