Well, it's a bit complicated, but it works like this.
Presuming you own a legit copy of the game, you can use any of the files that came with the game as a base for your own modifications, provided you credit Bethseda. It's even possible to do so for profit, but Bethseda still must be credited, and depending on the type of mod, you might have to work out commercial rights with Bethseda themselves, especially if your mod is a 'Total Conversion" mod that uses many heavily edited licensed assets, but there haven't been many cases of this actually happening, so I don't know the legal specifics.
If you use all original assets not connected to the base files of the Bethseda game, you have a lot more leeway since all you are really doing is using their engine to render the content. In this case, you'd have the strongest case to sue since you own the mod contents in their entirety.
About the only time I've seen Bethseda get downright edgy over copyright and commercial rights is people attempting to source port their products, like in the case of the DaggerXL engine and OpenMW.
The former isn't really an issue, since Bethseda themselves lost the source code to Daggerfall and thus the DaggerXL engine basically has to build everything to duplicate the original executable did from scratch. OpenMW was a bit of a touchier issue, since Bethseda still had the source code for Morrowind, but the OpenMW devs smoothed that over by specifying to Bethseda it was a free, open source program that used no executable code from the original game, and it would not be available for any platform save desktop/laptop computers (they were forbidden to port to any form of console, or stuff like Android). In the case of both engines, neither dev team charges a dime for them, and you still have to purchase a legally licensed copy of the games in question from Bethseda to use the engines in the first place.