Complex sentence: Lisa tries to let Nelson know how she feels about him by getting Milhouse to pass a love note to him in class; however the plan backfires, with Nelson seriously injuring Milhouse, thinking the note came from him.
Split into simpler sentences: Lisa tries to let Nelson know how she feels about him by getting Milhouse to pass a love note to him in class. However, the plan backfires, with Nelson seriously injuring Milhouse, thinking the note came from him.