Complex sentence: Artie Abrams (Kevin McHale) confronts Brittany: he becomes upset that Brittany cannot recognize that she is cheating on him with Santana, and calls her stupid.
Split into simpler sentences: Artie Abrams (Kevin McHale) confronts Brittany about the true nature of her relationship with Santana. He gets angered that Brittany does not understand that she is cheating, and calls her stupid.