What happened? 
( What ( happened ? ) )
(ROOT (SBARQ (WHNP (WP What)) (SQ (VP (VBD happened))) (. ?)))
What happened last night?
( What ( ( happened ( last night ) ) ? ) )
(ROOT (SBARQ (WHNP (WP What)) (SQ (VP (VBD happened) (NP (JJ last) (NN night)))) (. ?)))