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 will never forget this explanation even though I couldn't corroborate it.
( I ( ( ( will never ) ( ( forget ( this explanation ) ) ( even ( though ( I ( ( could n't ) ( corroborate it ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP I)) (VP (MD will) (ADVP (RB never)) (VP (VB forget) (NP (DT this) (NN explanation)) (SBAR (RB even) (IN though) (S (NP (PRP I)) (VP (MD could) (RB n't) (VP (VB corroborate) (NP (PRP it)))))))) (. .)))