When was that?
( When ( ( was that ) ? ) )
(ROOT (SBARQ (WHADVP (WRB When)) (SQ (VBD was) (NP (DT that))) (. ?)))
When did that happen?
( When ( ( ( did that ) happen ) ? ) )
(ROOT (SBARQ (WHADVP (WRB When)) (SQ (VBD did) (NP (DT that)) (VP (VB happen))) (. ?)))