Take another look at the sentence:

For his first date with Malinda, Dennis bought not only a dozen roses but he also purchased a box of chocolates.

When you use not only ... but also, you must maintain parallel structure. A dozen roses follows not only, so a box of chocolates is all that should come after but also.

He purchased a box of chocolates is a main clause and thus wrecks the parallelism.

Go to the next sentence.

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