Complex sentence: Indeed, he seems to have thought that the Old Ukrainian language stood in relation to modern Russian in a way similar to that of Old Czech to modern Polish or modern Slovak; that is, that one influenced but was not the same as the other.
Split into simpler sentences: Indeed, he seems to have thought that the Old Ukrainian language stood in relation to modern Russian in a way similar to that of Old Czech to modern Polish or modern Slovak. That is, that one influenced but was not the same as the other.