Review the item:
Not only did Dennis
apply an extra layer of deodorant to his armpits,
but he
also polished
each tooth with special care. He did not want to offend Malinda with any
unpleasant odors.
When you use not only ... but also, you must maintain parallel structure. Since Dennis did apply ..., a main clause, follows not only, a main clause must also follow but also. He polished ... qualifies, so the item is parallel.