i know i should n't have laughed , but 
(ROOT (S (S (NP (PRP i)) (VP (VBP know) (SBAR (S (NP (PRP i)) (VP (MD should) (RB n't) (VP (VB have) (VP (VBN laughed)))))))) (, ,) (CC but)))
(ROOT (S (S (NP (PRP )) (VP (VBP ) (SBAR (S (NP (PRP )) (VP (MD ) (RB ) (VP (VB ) (VP (VBN )))))))) (, ) (CC )))