Complex sentence: Märta Helena Reenstierna was the spouse of Cavalry Captain Christian Henrik von Schnell (d.1811) and had eight children, who all died as children except her son Hans Abraham, who died in a riding accident in 1812.
Split into simpler sentences: Märta Helena Reenstierna was the spouse of Cavalry Captain Christian Henrik von Schnell (d.1811). She had eight children, among whom only her son Hans Abraham became adults; he died in a riding accident in 1812.