Complex sentence: Karna became interested in the art of warfare and approached Dronacharya, who was an established teacher and taught the Kuru princes, but refused to take Karna as his student, since Karna was not a Kshatriya.
Split into simpler sentences: Karna became interested in the art of warfare and approached Dronacharya, an established teacher who taught the Kuru princes. But he refused to take Karna as his student, since Karna was not a Kshatriya.