As with other correlative conjunctions, you should follow each part of the not only … but also … construction with an element of the same grammatical type. Thus, instead of She not only bought a new car but also a new lawnmower, you should write She bought not only a new car but also a new lawnmower, because in this version both not only and but also are followed by noun phrases.
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Leaving out the also from this construction tends to intensify the first part of the construction rather than supplement it: She is not only smart but brilliant. He not only wanted the diamond but wanted it desperately.
