If Harry has seen his mother die, as revealed in The Deathly Hallows, why couldn't he see the thestrals before The Order of the Phoenix (after he's seen Cedric Diggory die)?
If Harry has seen his mother die, as revealed in The Deathly Hallows, why couldn't he see the thestrals before The Order of the Phoenix (after he's seen Cedric Diggory die)?