I think want to take the TOPIK exam someday 나는 언젠가 TOPIK 시험을 치고 싶다고 생각한다 -> this text is not wrong. but, I think 'think' is Invalid syntax my recommend text is I want to take the TOPIK exam someday 나는 언젠가 TOPIK 시험을 치고 싶다

1. I think want to take the TOPIK exam someday 언젠가 한국어능력시험을 보고 싶어요 (보고 싶다고 생각해요).. It means that you don't know what are you want actually. So you you aren't sure do you want to take exam or not.. So this is little bit strange. So I guess you're meaned this: I'm thinking to take the TOPIK exam someday 2. I'm thinking to take the TOPIK exam someday (저는) 언젠가 한국어능력시험을 볼까 생각중입니다. 3. I think I will take the TOPIK exam: 저는 언젠가 한국어능력 시험을 볼거라고 생각합니다 I'm not actually good at English, sorry~~