Let's say I want to copy/use a book published 200 years ago. It's definitely in the public domain. Someone publishes a new version of the book (the original copyright holder, if that matters). The new edition is exactly the same but with some grammatical corrections. Can I use a copy of this new edition - or do the grammatical changes and updates make it a new copyrighted work?
I know if I could obtain a copy of the old version it would be in the public domain.
The question is whether the new edition has enough originality to be considered a derived work. If it does, then you can't use it without a license. If not, you can use it freely.
The bar for originality is set very low, but it does exist. Applying mechanical changes to places where they are obviously needed is not going to qualify. If some sentences were rewritten in the process of fixing grammatical errors, that could be original enough.
If you can find what sort of changes were made, you might want to consult a copyright lawyer to see if they create a derivative work. We can't tell you yes or no here.
The creator of the derivative work has copyright in the derivative work. The copyright would protect only the new elements of the derivative work. Wikipedia is a good place to start.
In the case of a book with updated grammar, depending on the extent of the changes, it would probably be easier to copy the original directly than to eliminate the updates from a copy of the derivative work.
