what is the purpose of ‘것인’ in this sentence?
@Lucwilson - 것 means “thing”, and 이다 is the verb “to be”.
그게 당신 것인 줄 알았어요 means “I thought it was yours” or “I thought it belonged to you”.
You can learn more about this grammar pattern here: Korean Grammatical Forms
Thank you very much for the help!