Um, I'm sure I have I just can't think of what.
( Um ( , ( I ( ( 'm ( sure ( I ( have ( I ( just ( ( ca n't ) ( think ( of what ) ) ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (ADVP (RB Um)) (, ,) (NP (PRP I)) (VP (VBP 'm) (ADJP (JJ sure) (SBAR (S (NP (PRP I)) (VP (VBP have) (S (NP (PRP I)) (ADVP (RB just)) (VP (MD ca) (RB n't) (VP (VB think) (PP (IN of) (NP (WP what))))))))))) (. .)))
I am certain I never have done that.
( I ( ( am ( certain ( I ( never ( have ( done that ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP I)) (VP (VBP am) (ADJP (JJ certain) (SBAR (S (NP (PRP I)) (ADVP (RB never)) (VP (VBP have) (VP (VBN done) (NP (DT that)))))))) (. .)))