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've not been able to corroborate this explanation because everyone else left the island.
( I ( ( ( 've not ) ( ( been ( able ( to ( corroborate ( this explanation ) ) ) ) ) ( because ( ( everyone else ) ( left ( the island ) ) ) ) ) ) . ) )
(ROOT (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)))))) (SBAR (IN because) (S (NP (NN everyone) (RB else)) (VP (VBD left) (NP (DT the) (NN island))))))) (. .)))