I dated this person for a year.
Charm. They will make you feel very special at first. That's how they lure you in.
Manipulation. [google.com]. This article explains what this means.
Gaslighting. They will make you question your own reality and thoughts about their bad behaviors, using manipulation mentioned in bullet point #2.
Degrading. They will degrade you, subtly. Personal example; Me: "I'm working on getting my associates degree. I'm almost done" (I'm 27). Them: When I was your age, I was already in grad school working on getting my PhD.
Monopolizing the conversations. They talk at you, not with you. Example; Telling them a traumatic event you went through. They don't listen or acknowledge what you said. They jump right into talking about a traumatic event THEY went through.
Constant need for admiration.
They think they're better than you. They exhibit signs of arrogance and they're judgemental.
A narcissistic injury: They have low self esteem. "Narcissists react negatively to perceived or real criticism or judgment, boundaries placed on them, and/or attempts to hold them accountable for harmful behavior." When this happens, they might even discard you...thats how fragile their ego is.
Lack of sympathy or remorse. When you tell them that they've hurt you, they will not own up to it and will not say sorry. If they do say sorry in a rare case, it's manipulation.
They do not care about you, overall. They are usually incapable of this. They put themselves first and you're simply just a pawn to them.
I like to spread awareness about this and if I help even one person, I know that I have paid it forward in the sense of saving people from being in a relationship like this for long periods of time. Being in a relationship like this will destroy you. It did for me.
They are people too and are probably hurting on the inside. They have dealt with their own traumas themselves, BUT it is not your job to fix them. It rarely ever works. Love doesn't have to hurt.