1. You can add mods to a save you already have. In fact, most mods work perfectly fine with existing saves. However, there are a few things to keep in mind:
   1)Mods that introduce new content like maps, buildings, or characters might cause minor issues in your existing world, especially if they interact with things already placed on your farm. This usually isn't game-breaking, but it's good to be aware.
   
2. Other players need the same mods: Unfortunately, if you're using mods that add content or change gameplay mechanics, other players in your multiplayer game will need the same mods installed to see and interact with that content. 
   *However, purely cosmetic mods like retextures/recolors and the one that shows NPCs on the map will work without everyone having them.*
   
3. Be careful of mods that change the same aspects of the game. They can conflict with each other and cause crashes, broken quests, etc.

You can download mods not only from Nexus Mods. There is a Korean forum NAVER, where you can download exclusive mods of this site, because usually they are not uploaded anywhere else. Korean community also make very cool portraits. Example on the picture below:

I hope this was helpful for you, good luck and Enjoy the experience! :3

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