텔레비전에서 보던 빅벤을 실제로 보니 가슴이 설레더라. When I saw Big Ben which I used to see in TV with my own eyes, my heart fluttered. 가슴이 설레더라 << 가슴이 설레(다) + 더라 가슴이 설레다 : heart flutters -더라 used for just delivering new experienced fact to listener. -었더라 is used for emphasizing that the event was finished e.g. 인천 영종도 공항신도시에서 영화촬영을 하더라.(I'm not sure the event is finished or not) 나 빼놓고 다 출근했더라.(I'm sure the event was finished) 보던 << 보(다) + 던 보다 : to see 던 is used for unfinished action 었던 is used for finished action e.g. the apple I ate (unfinished) YESTERDAY = 어제 먹던 사과 the apple I ate (finished) YESTERDAY = 어제 먹었던 사과 보니 << 보다 + (으)니 보다 : to see (으)니 = (으)니까 There are two usages of the particle (으)니까 1. A/V+(으)니까 : Because of A/V, V is present or past tense 오늘 비가 많이 오니까 만나지 맙시다. Let's not meet today because of the heavy rain. 오늘은 날씨가 추우니까 따뜻한 옷을 입으세요. It's cold today, so put on warm clothes. 시간이 없으니까 빨리 먹자. Let's eat quickly because we don't have enough time. 2. V+(으)니까 : V and then, V is present tense 회의를 마치니까 12시였어요. When I finished the meeting, it was 12 o'clock. 철수에게 전화하니까 통화중이었어. When I called Chul-Su, his line was busy. 이 약을 먹으니까 감기가 나았어요. When I took this medicine, my cold got better. (으)니까 is emphasized expression of (으)니 and more commonly used in colloquial language. ~(으)니 is more commonly used in literally.

Heesob has corrected the typos for you: 텔레비전에서 보던 빅벤을 (실제)로 보니 가슴이 (설레)더라. No, "보던" is not the same as (으)ㄹ 때. 보던 is something you've seen. -더라 is relaying something you've seen/experienced/heard to the other person. 보니 가슴이 설레 = at that instant when you saw it, your heart jumped (you're very happy). Please read Heesob Park's answer, it's very detailed, and very good.