In terms of locations, で indicates the location where an action takes place, while に is either a direction, or a location in which something exists.|
It can help to think of it as depending on what class of verb you're using. If the verb is an action (eating, speaking, jumping, working etc.) then the place here it happens is indicated with で. If the verb is a movement - walking, flying, travelling, going - then the objective of that movement is shown with に. If the verb indicates position (ある and いる, basically), then the position is shown with に.
That basic rule does get a bit fucked up when you consider that some actions are movements that happen in a place, rather than towards a place - like walking in a park, for instance - and I think in some instances like that, を is the correct particle, just to make things even more fun. However, the above works as a general rule.
One thing to avoid is trying to find a logical "theory of everything" to explain the multiple uses of different particles. Yes, で indicates the object/method used to do something and also the location in which something happens, but it's better for your mental health to consider it to be a different particle with the same pronunciation in those two instances. This is particularly important for に, which can mean about a dozen different things depending on context - if you approach it by trying to learn off all the possible meanings of に, you'll drive yourself mad.
Shinji 5,903 posts
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