A soft fork would still be able to merge changes in the upstream project and then add their own changes on top. The most basic example would be a soft fork that only changes the default search engine - everything else is the same.

A hard fork - as I understand it - means the development takes a new direction and integrating the original upstream code becomes more difficult as projects diverge, to the point where they are basically incompatible with each other.

I see! Thank you.