‘-으니까/니까’ is used to show the discovery or realization of someone after the action has happened. It is used at the end of a verb, and the subject in the first clause is a first person, while the subject in the second clause is usually a third person.
● 밖을 보니까 비가 오고 있었어요. (When I looked outside, it was raining.)
● 셔츠를 입어 보니까 좀 작았어요. (When I wore the shirts, it was a little small.)