( It ( ( ( is not ) ( understood ( why ( there ( ( are only ) ( ( ( six quarks ) and ) ( six leptons ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP It)) (VP (VBZ is) (RB not) (VP (VBN understood) (SBAR (WHADVP (WRB why)) (S (NP (EX there)) (VP (VBP are) (ADVP (RB only)) (NP (NP (CD six) (NNS quarks)) (CC and) (NP (CD six) (NNS leptons)))))))) (. .)))
It is not understood why there are only six quarks and six leptons.
(ROOT (S (NP (EX There)) (VP (VBP are) (NP (NP (CD six) (NNS types)) (PP (IN of) (NP (NNS leptons))))) (. .)))