How can I relate my undergrad works to professors research areas?
Answering this question is hard for me because I don't know how to define a specific direction that doesn't make me limited and lose the opportunity of working with him.


This is a thoughtful response on his part. He is not asking for you to send him a dissertation proposal, but maybe something four or five times as broad. Maybe something like "I'm interested in interventions to support learning in recent immigrants to [your country]," rather than "The effect of X intervention on mathematical learning on late teenage refugees to [your country] who did not receive formal education in their home country."

It's not a quiz to see if you come up with a good enough one and "lose" the opportunity to work with him (unless he's a nasty person). More of a determination whether you two are a good scientific fit.