@LikeDuhWhatever the thing with abuse is it can be hard to spot at first. I say this because when you first encounter it, it catches you off guard. It is inconceivable that it just happened. And why?? Because people who are normal and non abusive judge the person who does the abusing by their own standards.
"I wouldn't do a thing like that, surely they just didn't" - that kind of mentality. Also being a normal person, you try to show you're partner you are willing to consider their feelings and you find ways to either a) rationalise their behaviour or b) fix your own so you don't offend again as you want to be a nice, considerate, good partner or c) a combo of both. The first time it happens can be a shock to the system. It blindsides you.
Then, the abuser knows it has you in his/her trap. They deploy different techniques for different scenarios. No two scenarios are ever the same so you don't know it is coming. You can be doing something normal and innocent. The abuser will look for ways to make you pay for 'it'. The abuser resorts to methods such as shouting, swearing, threats, intimidation. Or financial control. Or isolating you from friends or family. Or putting you down and belittling you and playing on your insecurities. Or the silent treatment. Or sexual abuse. Or physical abuse.
Eventually you get to a point that you don't know exactly WHAT is coming, but you are always on your guard waiting for the next thing. And it sure as hell comes and seemingly out of nowhere. It could be about anything and nothing. The abuser is always in a heightened state of being on the look out for things to throw at you. I'm not sure they know they are doing it. As time goes on, you become numb to it but also modify your behaviours so much that become unrecognisable to yourself. It is hard to leave, that's why so many stay longer than they should and often go back to their abusers.
If you've never been through it, you can't possibly know what it is like. Your are lucky.