Complex sentence: The first few years of their marriage were reasonably happy, leading to the birth of their daughter Vibeke in 1895, but as a result of P.S. Krøyer's periods of mental illness, by the early 1900s they spent ever more time apart.
Split into simpler sentences: The first few years of the marriage were reasonably happy, leading to the birth of their daughter Vibeke in 1895. However, as a result of P.S. Krøyer's periods of mental illness, by the early 1900s they spent ever more time apart.