Prepositions of time in English help indicate when something occurs. The primary prepositions include "at," "on," and "in." Use "at" for specific times, like "at 3 PM" or "at noon." "On" is for days and dates, such as "on Monday" or "on January 1st." Use "in" for longer periods, such as months, years, centuries, or non-specific times, like "in July," "in 2020," or "in the morning."