I mustn't keep you.
( I ( ( ( must n't ) ( keep you ) ) . ) )
(ROOT (S (NP (PRP I)) (VP (MD must) (RB n't) (VP (VB keep) (NP (PRP you)))) (. .)))
I should not keep you.
( I ( ( ( should not ) ( keep you ) ) . ) )
(ROOT (S (NP (PRP I)) (VP (MD should) (RB not) (VP (VB keep) (NP (PRP you)))) (. .)))