Why does Nelly not recognize Heathcliff when she first encounters him lurking in the garden of Thrushcross Grange?
First of all, it is nearly dark when Heathcliff hears a voice from behind her. The voice says, "Nelly, is that you?" Nelly says,
"It was a deep voice, and foreign in tone; yet there was something in the manner of pronouncing my name which made it sound familiar."
Nelly still can't really see who it is. All she sees is "a tall man dressed in dark clothes, with dark face and hair." Still, Heathcliff is so changed she does not recognize him as he approaches. A ray fell on his features; the cheeks were sallow, and half covered with black whiskers; the brows lowering, the eyes deep-set and singular.
The Nelly sees the eyes and remembers who she is speaking to. She exclaims,
"“What!” I cried, uncertain whether to regard him as a worldly visitor, and I raised my hands in amazement.
“What! you come back? Is it really you? Is it?”"
She cannot believe that Heathcliff has returned and his appearance and voice have changed since she last saw him. If is only when she sees his eyes that she recognizes him.