Complex sentence: Tidemand-Johannessen was the editor of the magazine "Avant-garden", published by the youth division of the Communist Party of Norway until the liberation from German occupation in 1945, and continued in this position when the magazine became legal after the war.
Split into simpler sentences: Tidemand-Johannessen was editor-in-chief of the magazine "Avant-garden", which was published by the youth league of the Communist Party of Norway until the liberation from German occupation in 1945. He continued in this position when the magazine became legal after the war.