Complex sentence: On October 20 1919 he married his wife Ida, with whom he had his only child, Olavi Johannes Lahti who later was a pilot in the Finnish Air Force and died in 1944.
Split into simpler sentences: On October 20 1919 he married his wife, Ida, with whom he had his only child. His son, Olavi Johannes Lahti, was later a pilot in the Finnish Air Force and died in 1944.