I'm confused.
( I ( ( 'm confused ) . ) )
(ROOT (S (NP (PRP I)) (VP (VBP 'm) (ADJP (VBN confused))) (. .)))
I don't understand.  
( I ( ( ( do n't ) understand ) . ) )
(ROOT (S (NP (PRP I)) (VP (VBP do) (RB n't) (VP (VB understand))) (. .)))