[Nyororin]

Originally Posted by godwine

Thanks..

but for the first 2 example, why can't it be

図書館で行く
公園に遊ぶ

When do you use NI and when do you use DE?

I`ve always sort of accepted に as along the lines of "to / by" and で as "in/within". You wouldn`t really use one in place of the other.