What happened? 
( What ( happened ? ) )
(ROOT (SBARQ (WHNP (WP What)) (SQ (VP (VBD happened))) (. ?)))
I know what happened
( I ( know ( what happened ) ) )
(ROOT (S (NP (PRP I)) (VP (VBP know) (SBAR (WHNP (WP what)) (S (VP (VBD happened)))))))