( Density ( ( is ( ( ( the amount ) ( of ( ( stuff or ) matter ) ) ) ( in ( a ( certain space ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Density)) (VP (VBZ is) (NP (NP (DT the) (NN amount)) (PP (IN of) (NP (NN stuff) (CC or) (NN matter))) (PP (IN in) (NP (DT a) (JJ certain) (NN space))))) (. .)))
Density is the amount of stuff or matter in a certain space.
(ROOT (S (NP (NNP Density)) (VP (VBZ describes) (SBAR (WHADJP (WRB how) (JJ much)) (S (NP (NN matter)) (VP (VBZ is) (PP (IN in) (NP (NP (DT a) (JJ certain) (NN amount)) (PP (IN of) (NP (NN space))))))))) (. .)))