Complex sentence: Gandalf deduces that Sauron will attack Gondor's capital Minas Tirith, so he rides there to warn them, taking Pippin with him because Sauron thinks Pippin is the ring bearer.
Split into simpler sentences: Gandalf deduces that Sauron will attack Gondor's capital Minas Tirith. He rides there to warn them, taking Pippin with him because Sauron believes Pippin is the ring bearer.