( Clouds ( ( form ( by ( ( the condensation ) ( of ( water vapor ) ) ) ) ) . ) )
(ROOT (S (NP (NNS Clouds)) (VP (VBP form) (PP (IN by) (NP (NP (DT the) (NN condensation)) (PP (IN of) (NP (NN water) (NN vapor)))))) (. .)))
Clouds form by the condensation of water vapor.
(ROOT (S (NP (NNP Condensation)) (VP (MD must) (VP (VB occur) (PP (IN for) (NP (NNS clouds))) (S (VP (TO to) (VP (VB form)))))) (. .)))