There are many usages of "니" but it is the most commonly used as a question marker and a conjunctive particle. I would say if a sentence ends with "니", it is a question marker in almost a hundred percent of cases. However, if another clause comes after "니", it is a conjunctive particle.
봄이 오니 꽃이 핀다. Spring came, then flowers are blooming.
서울에 도착하니 7시였다. When I arrived at Seoul, it was 7 o'clock.
If you have seen any other examples that don't fall under these usages, please let me know. I will explain again.