Complex sentence: During the honeymoon, when Yeats was thinking about Iseult's rejection of him, she began the automatic writing which fascinated him and he wrote about days later in what was to be "A Vision" and was to keep together the marriage for many years.
Split into simpler sentences: During the honeymoon, when Yeats was thinking about Iseult's rejection of him, she began the automatic writing which fascinated him. He wrote about it days later in what was to be "A Vision", and it held the marriage together for many years.