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 has to stay above a certain level.
( ( The ( DO concentration ) ) ( ( has ( to ( stay ( above ( a ( certain level ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT The) (NNP DO) (NN concentration)) (VP (VBZ has) (S (VP (TO to) (VP (VB stay) (PP (IN above) (NP (DT a) (JJ certain) (NN level))))))) (. .)))