Complex sentence: Alek and Deryn get into an argument about her lie to Alek, and, after she leaves, Alek realises that Deryn didn't tell him because she is in love with him.
Split into simpler sentences: Alek and Deryn get into an argument about her lie to Alek. After she leaves, Alek realises that Deryn didn't tell him because she is in love with him.