Don't remember. 
( ( ( Do n't ) remember ) . )
(ROOT (S (VP (VB Do) (RB n't) (VP (VB remember))) (. .)))
I can't remember.
( I ( ( ( ca n't ) remember ) . ) )
(ROOT (S (NP (PRP I)) (VP (MD ca) (RB n't) (VP (VB remember))) (. .)))