They didn't answer.
( They ( ( ( did n't ) answer ) . ) )
(ROOT (S (NP (PRP They)) (VP (VBD did) (RB n't) (VP (VB answer))) (. .)))
They could not respond because they were gagged.
( They ( ( ( could not ) ( respond ( because ( they ( were gagged ) ) ) ) ) . ) )
(ROOT (S (NP (PRP They)) (VP (MD could) (RB not) (VP (VB respond) (SBAR (IN because) (S (NP (PRP they)) (VP (VBD were) (VP (VBN gagged))))))) (. .)))