Elena: What am I to you? Who am I to you?
Stefan: You are not Katherine. You are the opposite of everything that she was.
Elena: And when did you figure that out? Before you kissed me? Before we slept together?
Stefan: Before I met you.
Stefan: The first day of school, when we met, that wasn’t for the first time, Elena.
Elena: Then when was it?
Stefan: May 23rd, 2009.
Elena: But that was…
Stefan: That was the day your parents’ car went off the bridge.
Elena: You were there?
Stefan: Every couple of years I come back here to see Zack and see my home. And last spring I was out in the woods by Old Wickard Bridge and I heard the accident. All of it. I was fast getting there but not fast enough. The car was already submerged. Your dad, he was still conscious when I was able to get to him but he wouldn’t let me help him… until I helped you.
Elena: Oh my god. (cries) When I woke up in the hospital, no one could figure how I got out of the car. They said it was a miracle.
Stefan: I went back for them but it was too late. I couldn’t… I couldn’t safe them. When I pulled you out, I looked at your face and you looked like Katherine I couldn’t believe the resemblance. After that I spent months making sure that you weren’t her. I watched you. I learned everything that I could about you. And I saw that you were nothing like Katherine. And I wanted to leave town but Elena, I couldn’t… I couldn’t leave without knowing you. I’m so sorry that I didn’t tell you.
The Vampire Diaries 1.11 – “Bloodlines”