I just can't remember.
( I ( just ( ( ( ca n't ) remember ) . ) ) )
(ROOT (S (NP (PRP I)) (ADVP (RB just)) (VP (MD ca) (RB n't) (VP (VB remember))) (. .)))
I am having trouble remembering.
( I ( ( am ( having ( trouble remembering ) ) ) . ) )
(ROOT (S (NP (PRP I)) (VP (VBP am) (VP (VBG having) (NP (NN trouble) (VBG remembering)))) (. .)))