The document discusses the proper use of the prepositions "at", "in", and "on" when indicating time. It provides examples of when to use each preposition, such as using "at" for specific clock times or dates, "in" for months or periods of time, and "on" for specific days. It also includes an exercise for students to practice identifying the correct preposition based on time-related phrases. The student received a poor grade on the exercise, incorrectly answering the majority of the questions.