Go ahead.
( ( Go ahead ) . )
(ROOT (S (VP (VB Go) (ADVP (RB ahead))) (. .)))
They were given the go ahead.  
( They ( ( were ( given ( ( the go ) ahead ) ) ) . ) )
(ROOT (S (NP (PRP They)) (VP (VBD were) (VP (VBN given) (ADVP (NP (DT the) (NN go)) (RB ahead)))) (. .)))