Complex sentence: Shōju assigned him a series of "hard-to-pass" koans, which lead to three occasional moments of satori, but it was only eighteen years later that Hakuin really understood what Shōju meant with this.
Split into simpler sentences: Shōju assigned him a series of "hard-to-pass" koans. These led to three isolated moments of satori, but it was only eighteen years later that Hakuin really understood what Shōju meant with this.