The document discusses inversion in English grammar. Inversion means putting the verb before the subject. It is commonly used in questions by moving the verb in front of the subject. There are also some other situations where inversion is used, including with negative adverb phrases at the beginning of sentences to add emphasis, in conditional sentences using "had" or "were", after adverbial phrases of place, and after expressions like "so...that". Inversion changes the typical subject-verb word order in a sentence for specific grammatical purposes or effects.