( ( If ( you ( use ( a rock ) ) ) ) ( , ( it ( ( needs ( to ( be ( at ( groin height ) ) ) ) ) . ) ) ) )
(ROOT (S (SBAR (IN If) (S (NP (PRP you)) (VP (VBP use) (NP (DT a) (NN rock))))) (, ,) (NP (PRP it)) (VP (VBZ needs) (S (VP (TO to) (VP (VB be) (PP (IN at) (NP (NN groin) (NN height))))))) (. .)))
If you use a rock, it needs to be at groin height.
(ROOT (S (NP (NP (DT A) (NN wall)) (PP (IN of) (NP (NP (NNS rocks)) (CC or) (NP (JJ concrete) (DT a))))) (VP (VBD called) (NP (DT a) (NN groin))) (. .)))