when will they find out that katherine is in elena's body
No Exit (The Vampire Diaries) Katherine (Nina Dobrev), still pretending to be Elena, tries to get closer to Stefan (Paul Wesley) so they can be back together. When Caroline (Candice Accola) arrives and tells them about Damon, Stefan wants to find him and Katherine offers to go with him. Caroline warns him that it is weird for him to be so close to "Elena" after everything that happened but he reassures her that they are just friends. Before Stefan and Katherine go, Caroline gets a phone call from Tyler (Michael Trevino) who tells her that he worries about Matt (Zach Roerig), who is vanished for two days, and that Nadia (Olga Fonda) was compelling him. Katherine hears Caroline's conversation with Tyler and she calls Nadia to ask if everything is fine. When Nadia tells her that Matt found out that she is not Elena, Katherine asks her to kill him. Caroline leaves to go to Tyler's while Katherine and Stefan leave to find Damon.
Elena Gilbert In the final episode, Elena makes an appearance. When a weakened Bonnie dies from the sounding bell, she meets Elena in a dream-like forest. Elena hugs her and wonders if she really has died. Enzo appears and says it isn't her time and takes Bonnie back to the world of the living. When Bonnie wakes up, it was Stefan who resuscitated her. Elena, asleep, is then taken to the Mystic Falls High School by Katherine and places her in the boiler room. Stefan tries to rescue Elena, but Katherine had Kai spell the boiler room, so that Elena can't be taken out of the room. After Bonnie had deflected the hellfire back into the bell, Bonnie passes out. Elena wakes up and wanders the school. Stefan meets up with her. It looks like Bonnie has died, but in fact Elena is still sleeping and Stefan's ghost came to visit her in her dream. He had taken the place of killing Katherine instead of Damon, since Damon is human after Stefan gave him the cure. He came to say goodbye. After a tearful hug and saying goodbye, Stefan goes into afterlife. Elena finally wakes up because Bonnie has broken the spell over her. Elena reunites with Damon and the rest of the gang. Elena gives Caroline the message Stefan whispered into her ear, saying he did hear her voice message and will love her forever. In the epilogue, Elena continued to studying medical school. After finishing her education, she went back to Mystic Falls and lived together with Damon, happily married for a long time. After living out her life, she crosses over to the afterlife where she finally reunites with her parents, Aunt Jenna and Uncle John.