Prepositions are short words like "on", "in", and "to" that are used before nouns or gerund verbs to indicate location, direction, or time. Even advanced English learners find prepositions difficult because the meaning depends on context. There are few rules for using prepositions correctly. The document provides tables of common English prepositions and their usage for time and place to help learners understand when to use different prepositions like "on", "in", "at", "by", etc. Mastering prepositions requires looking up words in a dictionary, extensive reading, and memorizing phrases.