Complex sentence: He assisted her in carrying out the resolution, and he gave evidence of this statement before the magistrates but prefaced his evidence by a declaration that he must withhold any further information on the ground that it had been received under the seal of "sacramental confession".
Split into simpler sentences: He assisted her in carrying out this resolution, and he gave evidence of this statement before the magistrates. But he prefaced his evidence by a declaration that he must withhold any further information on the ground that it had been received under the seal of "sacramental confession".