The idiom is "not X but Y". You are not allowed to insert an "are" in this idiom.
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Can someone please explain why you are not allowed to insert an 'are' in this idiom?
Here's an example:
Headache is not caused by x or y, but is by z (x,y,z are some nouns).
Why 'is' is not allowed here? I think that this structure maintains parallelism:
NOT verb by noun or noun, BUT verb by noun
Please explain.
Thanks a lot.