( Oxidation ( ( is ( ( a reaction ) ( ( in which ) ( oxygen ( combines ( with ( another substance ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Oxidation)) (VP (VBZ is) (NP (NP (DT a) (NN reaction)) (SBAR (WHPP (IN in) (WHNP (WDT which))) (S (NP (NN oxygen)) (VP (VBZ combines) (PP (IN with) (NP (DT another) (NN substance)))))))) (. .)))
Oxidation is a reaction in which oxygen combines with another substance.
(ROOT (S (NP (NNP Combustion) (NN reaction)) (VP (VBZ occurs) (SBAR (WHADVP (WRB when)) (S (NP (DT a) (NN substance)) (VP (VBZ reacts) (ADVP (RB quickly)) (PP (IN with) (NP (NN oxygen))))))) (. .)))