This is Susan.
( This ( ( is Susan ) . ) )
(ROOT (S (NP (DT This)) (VP (VBZ is) (NP (NNP Susan))) (. .)))
Susan is who this is. 
( Susan ( ( is ( who ( this is ) ) ) . ) )
(ROOT (S (NP (NNP Susan)) (VP (VBZ is) (SBAR (WHNP (WP who)) (S (NP (DT this)) (VP (VBZ is))))) (. .)))