The document discusses several structural issues with sentences:
1) Split infinitives occur when words are placed between "to" and the verb in an infinitive phrase, which should be avoided.
2) Dangling modifiers result when the subject of the sentence is not logically connected to an introductory phrase or clause.
3) Parallel structure creates emphasis and rhythm when elements in a series use the same grammatical form. Sentences should be edited to maintain parallelism.