Complex sentence: He was closely follwed by the Gestapo and was put away for seven months by the French police, after they learnt he was a painter, in a red cell in order to wear off his vision.
Split into simpler sentences: He was closely follwed by the Gestapo and arrested for seven months by the French police. After they learned he was a painter, he was put in a red cell in order to wear off his vision.