This might super dumb to ask ,but I really need some clear cut answer to this nightmare of mine

Due to this one operation I lost confidence even in questions that I am capable of solving and many Wrong Submissions during the contests where I am having trouble to use this mod (sounds stupid)

Just a simple question....

WHEN & WHERE(i.e in which part of the code) DO I NEED TO USE (% MOD)?

Are there any golden rules on where to use %mod

Also what is the guarantee (or any proof kinda thing) that using %mod at a particular place do not change my final answer??