I couldn't have done it.
( I ( ( ( could n't ) ( have ( done it ) ) ) . ) )
(ROOT (S (NP (PRP I)) (VP (MD could) (RB n't) (VP (VB have) (VP (VBN done) (NP (PRP it))))) (. .)))
Oh I could have done it if I wanted to.
( Oh ( I ( ( could ( have ( ( done it ) ( if ( I ( wanted to ) ) ) ) ) ) . ) ) )
(ROOT (S (ADVP (RB Oh)) (NP (PRP I)) (VP (MD could) (VP (VB have) (VP (VBN done) (NP (PRP it)) (SBAR (IN if) (S (NP (PRP I)) (VP (VBD wanted) (PP (TO to)))))))) (. .)))