sarahk wrote:How do I say
When I was younger,...
내가 어렸을 때,
내가 어렸을 적에,
If you already aged enough and try not to say about your childhood,
내가 젊었을 때,
내가 젊었을 적에,
Of course you could be more politer by using 제가 instead of 내가.
And for usakorjb03..
..는데 is a conjunction which connects two sentences.
It is used for giving some situations earlier in order to explain or to ask or to assign or to offer in the followed sentence.
For example,
길을 가고 있는데 비가 오기 시작했다.
I was walking on the street and the rain starts falling.
버스가 도착했는데 타시죠?
Why don't you ride on the bus while it has already arrived.
점심을 먹었는데 배가 고프다.
Although I ate lunch but I'm still hungry.
그렇다고 생각했는데 사실은 그렇지 않았다.
I thought it was but it wasn't actually.
There is another usage of this.
Sometimes it is used as an exclamation copula so that the speaker could expect some kinds of response from the listener.
For example,
너 오늘 참 예쁜데?
You look so beautiful today. (what's up?)
나 학교 안 갔는데.
I didn't go to school. (don't you ask me why?)
Hope this helps.