( ( When ( ( acids ( caused ( by pollutants ) ) ) ( combine ( with ( water ( in ( the air ) ) ) ) ) ) ) ( , ( ( acid rain ) ( ( is formed ) . ) ) ) )
(ROOT (S (SBAR (WHADVP (WRB When)) (S (NP (NP (NNS acids)) (VP (VBN caused) (PP (IN by) (NP (NNS pollutants))))) (VP (VBP combine) (PP (IN with) (NP (NP (NN water)) (PP (IN in) (NP (DT the) (NN air)))))))) (, ,) (NP (JJ acid) (NN rain)) (VP (VBZ is) (VP (VBN formed))) (. .)))
When acids caused by pollutants combine with water in the air, acid rain is formed.
(ROOT (S (NP (JJ Acid) (NN rain)) (VP (MD may) (VP (VB be) (VP (VBN caused) (PP (IN by) (NP (NN air) (NN pollution)))))) (. .)))