
It was from the last episode. We see that after they do sex, he says "I know everything about you" and it skips to the past. It was before she was hired, he calls the girl's brother and inquires about if she looking for a job. He is looking at her social media. He just received an strange email with the game with the favorability.
Also, it was a bit obvious since he only reacted when she acted like a big sister. So he liked her since they were kids and she took care of him, but he didn't like the sexy flirt she had become.

It was just a few panels, but I picked that up a little better than others since I majored in Creative Writing, so I always think what the authors tries to convey in each thing. After a while it becomes a habit and I don't need to think much to get it.
But yeah, I think it would have been easier to understand if they had more clues that this was on the past. For me it was clear because of the conversation about Noona looking for a job, but for others it might be a bit strange. I think the author was trying to be subtle, to show the ML's story without much narration or explanation.

Yeah, don't worry. It was like 4 panels, and the author didn't announced it was from the past.
If you want to understand better try practicing. If you see the panels changed scenario try pausing a bit and ask yourself these 3 questions: What time is it, still the current time, past or future? What are the charters' motivations? And, why is the author doing this?
After a while you won't need to ask yourself that and your brain will learn to think like that on its own. It will pick up many things. The brain is amazing.
And it's fine if you don't get it sometimes. It always go back a few panels back or reread a chapter if I feel I didn't quite understood it. But it helps. I end up explaining sometime in the comments because of that. But rereading is also a hobby of mine. I sometimes do it because I just loved the feelings I got from something. It makes me smile like an idiot.
It's also fine if you do not want to do what I said. It's just a suggestion. But no matter what, it is fine to be wring and ask questions.
i WISH this was a slowburn & he was more pathetic. this would’ve been way better if she started by figuring out how to grab his attention for a longer period of time. it kinda feels undeserved or something now.