they could n't have done in half an hour 
(ROOT (S (NP (PRP they)) (VP (MD could) (RB n't) (VP (VB have) (VP (VBN done) (PP (IN in) (NP (PDT half) (DT an) (NN hour))))))))
(ROOT (S (NP (PRP )) (VP (MD ) (RB ) (VP (VB ) (VP (VBN ) (PP (IN ) (NP (PDT ) (DT ) (NN ))))))))