I should worry.
( I ( ( should worry ) . ) )
(ROOT (S (NP (PRP I)) (VP (MD should) (VP (VB worry))) (. .)))
I need to be worried.
( I ( ( need ( to ( be worried ) ) ) . ) )
(ROOT (S (NP (PRP I)) (VP (VBP need) (S (VP (TO to) (VP (VB be) (ADJP (JJ worried)))))) (. .)))