they spend years trying to comprehend it 
(ROOT (S (NP (PRP they)) (VP (VBP spend) (NP (NNS years)) (S (VP (VBG trying) (S (VP (TO to) (VP (VB comprehend) (NP (PRP it))))))))))
(ROOT (S (NP (PRP )) (VP (VBP ) (NP (NNS )) (S (VP (VBG ) (S (VP (TO ) (VP (VB ) (NP (PRP ))))))))))