Think of your greatest accomplishments.
( ( Think ( of ( your ( greatest accomplishments ) ) ) ) . )
(ROOT (S (VP (VB Think) (PP (IN of) (NP (PRP$ your) (JJS greatest) (NNS accomplishments)))) (. .)))
Think of your greatest accomplishment, but only if it involves elderly horses. 
( ( Think ( ( of ( your ( greatest accomplishment ) ) ) ( , ( but ( only ( if ( it ( involves ( elderly horses ) ) ) ) ) ) ) ) ) . )
(ROOT (S (VP (VB Think) (UCP (PP (IN of) (NP (PRP$ your) (JJS greatest) (NN accomplishment))) (, ,) (CC but) (SBAR (RB only) (IN if) (S (NP (PRP it)) (VP (VBZ involves) (NP (JJ elderly) (NNS horses))))))) (. .)))