He slowed.
( He ( slowed . ) )
(ROOT (S (NP (PRP He)) (VP (VBD slowed)) (. .)))
He slowed down to let the horse by.
( He ( ( ( slowed down ) ( to ( ( let ( the horse ) ) by ) ) ) . ) )
(ROOT (S (NP (PRP He)) (VP (VBD slowed) (PRT (RP down)) (S (VP (TO to) (VP (VB let) (NP (DT the) (NN horse)) (PP (IN by)))))) (. .)))