The conversation he had overheard had stimulated his curiosity.
( ( ( The conversation ) ( he ( had overheard ) ) ) ( ( had ( stimulated ( his curiosity ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (NN conversation)) (SBAR (S (NP (PRP he)) (VP (VBD had) (VP (VBN overheard)))))) (VP (VBD had) (VP (VBN stimulated) (NP (PRP$ his) (NN curiosity)))) (. .)))
he hadn't heard anything
( he ( ( had n't ) ( heard anything ) ) )
(ROOT (S (NP (PRP he)) (VP (VBD had) (RB n't) (VP (VBN heard) (NP (NN anything))))))