I've not been able to corroborate this explanation, but I'll never forget it.
( ( ( ( ( I ( ( 've not ) ( been ( able ( to ( corroborate ( this explanation ) ) ) ) ) ) ) , ) but ) ( I ( ( 'll never ) ( forget it ) ) ) ) . )
(ROOT (S (S (NP (PRP I)) (VP (VBP 've) (RB not) (VP (VBN been) (ADJP (JJ able) (S (VP (TO to) (VP (VB corroborate) (NP (DT this) (NN explanation))))))))) (, ,) (CC but) (S (NP (PRP I)) (VP (MD 'll) (ADVP (RB never)) (VP (VB forget) (NP (PRP it))))) (. .)))
I corroborated this explanation, and will soon forget it.
( I ( ( ( ( ( corroborated ( this explanation ) ) , ) and ) ( ( will soon ) ( forget it ) ) ) . ) )
(ROOT (S (NP (PRP I)) (VP (VP (VBD corroborated) (NP (DT this) (NN explanation))) (, ,) (CC and) (VP (MD will) (ADVP (RB soon)) (VP (VB forget) (NP (PRP it))))) (. .)))