The document discusses run-on sentences and comma splices, which are two types of sentence errors that occur when two simple sentences are incorrectly joined together without proper punctuation or conjunctions. It provides examples of run-on sentences and comma splices, and explains the correct ways to separate sentences using periods or joining them using coordinating conjunctions like "and", "but", or "so" along with a comma. The key is to identify the individual simple sentences and either separate them with periods or connect them in a way that maintains the intended meaning.