The DO concentration must not fall below
( ( The ( DO concentration ) ) ( ( must not ) ( fall below ) ) )
(ROOT (S (NP (DT The) (NNP DO) (NN concentration)) (VP (MD must) (RB not) (VP (VB fall) (ADVP (RB below))))))
The DO concentration fell below.
( ( The ( DO concentration ) ) ( ( fell below ) . ) )
(ROOT (S (NP (DT The) (NNP DO) (NN concentration)) (VP (VBD fell) (ADVP (RB below))) (. .)))