There are 11 types of uses of prepositions you can see here: http://www.michigan-proficiency-exams.com/preposition-usage.html Mainly they are to help describe where something is or some issue with placement space or when something happens or some issue with time. These words are placed between two items or ideas to compare them. You can see how many prepositions there are and know I can not give you examples for all the ways they are used, but here are two to discuss. The book is BESIDE the chair. The event will be over BEFORE we get there. As you see in the placement and time examples the prepositions are between items or concepts. This is typical. Below are prepositions. Notice how many would be used for time and placement/space issues and could be used similarly to the examples above. aboard about above across after against along amid among anti around as at before behind below beneath beside besides between beyond but by concerning considering despite down during except excepting excluding following for from in inside into like minus near of off on onto opposite outside over past per plus regarding round save since than through to toward towards under underneath unlike until up upon versus via with within without

Prepositions can be complicated, so I'd recommend looking for grammar websites like the one Jesse listed.