This document discusses prepositions of time and provides examples of their uses. It explains that "in" is used to indicate a period of time, such as "in the morning". "At" is used to indicate a specific time, like "at 7PM". "On" is used to indicate when an activity is regularly done, such as "on Tuesdays". It concludes with an example that combines all three prepositions: "I usually do not have a class on Thursday, but in September, I always have make up classes replacing holidays at 10 AM."