Complex sentence: When Lyra and Will confront Sir Charles, he admits that he has stolen the alethiometer but makes an agreement with the pair: if they retrieve a certain knife from Cittàgazze and give it to him, he will return the alethiometer to Lyra.
Split into simpler sentences: When Lyra and Will confront Sir Charles, he admits that he has stolen the alethiometer. He makes an agreement with the pair that if they retrieve a certain knife from Cittàgazze and give it to him, he will return the alethiometer to Lyra.