"David only knows how to cook cabbage."
Translation:David ne sait cuisiner que le chou.
I had that as well. However, I think that would mean he only knows how to cook cabbage, and literally doesn't know how to do anything else. Whereas Je ne sait cuisiner que le chou means cabbage is the only thing he knows how to cook. While the English is ambiguous, it does seem far more likely that it means the second one (since I can't imagine a person surviving if the only knowledge they have is how to cook cabbage).