Complex sentence: They repeatedly ask him about the missing "Great Seal", but he knows nothing about it; however, when Tom is asked to sit in on judgments, his common-sense observations reassure them his mind is sound.
Split into simpler sentences: They repeatedly ask him about the missing "Great Seal", but he knows nothing about it. However, when Tom is asked to sit in on judgments, his common-sense observations reassure them his mind is sound.