He did not know that other aircraft had been hijacked.
( He ( ( ( did not ) ( know ( that ( ( other aircraft ) ( had ( been hijacked ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP He)) (VP (VBD did) (RB not) (VP (VB know) (SBAR (IN that) (S (NP (JJ other) (NN aircraft)) (VP (VBD had) (VP (VBN been) (VP (VBN hijacked)))))))) (. .)))
He didn't know about the plane hijacking.
( He ( ( ( did n't ) ( know ( about ( the ( plane hijacking ) ) ) ) ) . ) )
(ROOT (S (NP (PRP He)) (VP (VBD did) (RB n't) (VP (VB know) (PP (IN about) (NP (DT the) (NN plane) (NN hijacking))))) (. .)))