Complex sentence: Marcus was close to Tris's parents but Four never accompanied him to dinner when they were invited over this why he and Tris didn't remember each other but when Tris's mom, Natalie, came to visit her on visiting day, he looked familiar.
Split into simpler sentences: Marcus was close to Tris's parents, but Four never accompanied him to dinner when they were invited over. However, when Tris's mom, Natalie, came to visit her on visiting day, he looked familiar.