rishi.riddle wrote:which one idiom is correct? :-

not only X..... but also Y
OR
not only X..... but Y

Give the explanation for correct one and also for wrong one.............

Not Only X But Also Y is a correct idiom provided X and Y both provide same feel. This means X and Y cannot provide contrasting meaning but same kind of meaning.

If X is saying positive about something then Y must say positive about that thing.

The camera not only takes clear picture but also takes clear video.

here, both X and Y says positively about the camera feature so correct.

Not sure about Not Only X But Y, but for sure "Not X But Y" is correct.

In terms of idiom, they're both correct in GMAT. In fact, there are actual examples of both cases in the OG/GMAT Prep.

"not only X...but also Y..." is the most common form in GMAT, and I have only seen one case of "not only X...but Y..." and it was in GMAT Prep. However, there's a consensus that the latter of the two is incorrect, but personally I wouldn't base my answer on the difference between the two.