Here is the item:
For his first date with Malinda, Dennis bought
not only a dozen roses
but he
also purchased a box
of chocolates.
Nice choice! 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.