who know nothing about the subject 
(ROOT (SBAR (WHNP (WP who)) (S (VP (VBP know) (NP (NP (NN nothing)) (PP (IN about) (NP (DT the) (NN subject))))))))
(ROOT (SBAR (WHNP (WP )) (S (VP (VBP ) (NP (NP (NN )) (PP (IN ) (NP (DT ) (NN ))))))))