Once there, he or she must alight from the vehicle and proceed to the mailbox, then return to the vehicle, turn it around and proceed to the road.
( ( Once there ) ( , ( ( ( he or ) she ) ( ( must ( ( ( ( ( ( ( ( ( alight ( from ( the vehicle ) ) ) and ) ( proceed ( to ( the mailbox ) ) ) ) , ) ( then ( return ( to ( the vehicle ) ) ) ) ) , ) ( ( turn it ) around ) ) and ) ( proceed ( to ( the road ) ) ) ) ) . ) ) ) )
(ROOT (S (ADVP (RB Once) (RB there)) (, ,) (NP (PRP he) (CC or) (PRP she)) (VP (MD must) (VP (VP (VB alight) (PP (IN from) (NP (DT the) (NN vehicle)))) (CC and) (VP (VB proceed) (PP (TO to) (NP (DT the) (NN mailbox)))) (, ,) (VP (ADVP (RB then)) (VB return) (PP (TO to) (NP (DT the) (NN vehicle)))) (, ,) (VP (VB turn) (NP (PRP it)) (ADVP (RB around))) (CC and) (VP (VB proceed) (PP (TO to) (NP (DT the) (NN road)))))) (. .)))
They must get out of the vehicle to go to the mailbox.
( They ( ( must ( ( get out ) ( of ( the ( vehicle ( to ( go ( to ( the mailbox ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP They)) (VP (MD must) (VP (VB get) (PRT (RP out)) (PP (IN of) (NP (DT the) (NN vehicle) (S (VP (TO to) (VP (VB go) (PP (TO to) (NP (DT the) (NN mailbox)))))))))) (. .)))