I don't know.
( I ( ( ( do n't ) know ) . ) )
(ROOT (S (NP (PRP I)) (VP (VBP do) (RB n't) (VP (VB know))) (. .)))
I am certain.
( I ( ( am certain ) . ) )
(ROOT (S (NP (PRP I)) (VP (VBP am) (ADJP (JJ certain))) (. .)))