i don't know um-hum
( i ( ( do n't ) ( know um-hum ) ) )
(ROOT (S (NP (FW i)) (VP (VBP do) (RB n't) (VP (VB know) (NP (NN um-hum))))))
I am not certain.
( I ( ( ( am not ) certain ) . ) )
(ROOT (S (NP (PRP I)) (VP (VBP am) (RB not) (ADJP (JJ certain))) (. .)))