First, here's a material that made me keep thinking about this issue in the first place.
She acted as if she didn't know me.
It was the first time that I'd seen her since I was in elemantary school.(and now I'm 26). A couple of days ago, when I met her, she acted as if she didn't know me. I think she didn't remember me.
She acted as if she hadn't known me.
I broke up with her a couple of weeks ago, and when I met her yesterday, she acted as if she hadn't known me. She definitely knew who I am, but pretended as if he hadn't known me because she didn't want to talk to me.
Is it right that I make a situation like that with each sentence? I hope this try will be concerned not only for me, but also for other English learners.