Complex sentence: Ariana's magical ability was impaired when three Muggle boys spied her magic; they first tried to get her repeat what they had seen, and upon her failure to recreate it, they tried to stop her from being "different".
Split into simpler sentences: Ariana's magical ability was impaired when three Muggle boys spied her doing magic. Upon her failure to recreate it, they tried to stop her from being "different".