The document discusses parallel structure in sentences. It provides examples of sentences with parallel structures and identifies the grammatical forms being parallelized, such as present tense verbs, adverbs, etc. It also discusses parallelism with coordinators and correlatives conjunctions. Parallel structure means using the same grammatical forms in a sentence. The document then discusses problems like sentence fragments, choppy sentences, run-on sentences, comma splices, and stringy sentences, providing examples and solutions for each.