What happened? 
( What ( happened ? ) )
(ROOT (SBARQ (WHNP (WP What)) (SQ (VP (VBD happened))) (. ?)))
What went on?
( What ( ( went on ) ? ) )
(ROOT (SBARQ (WHNP (WP What)) (SQ (VP (VBD went) (PRT (RP on)))) (. ?)))