The document discusses the proper use of the prepositions "for", "during", and "while" when referring to time. It provides examples of how each preposition is used: "for" refers to a period of time, "during" refers to an event or activity, and "while" refers to two simultaneous actions or events. Common mistakes are highlighted, such as using "during" to refer to a length of time rather than an event. An exercise is included for the reader to practice using the three prepositions correctly in sentences referring to time.