Complex sentence: Shortly thereafter, FDR runs into Lauren at a video store and tries to flirt with her, not knowing she's Tuck's date, but she surmises that he's a ladies' man and ignores him.
Split into simpler sentences: Shortly thereafter, FDR runs into Lauren at a video store and tries to flirt with her, not knowing she's Tuck's date. She surmises that he's a ladies' man and ignores him.