Complex sentence: After attending the Polytechnical Secondary School he was an apprentice seaman (1973-1975) at the Deutsche Seereederei Rostock, which allowed him to travel to Sweden, the Netherlands and Cuba in his youth.
Split into simpler sentences: After attending the Polytechnical Secondary School he made a seaman's apprenticeship from 1973 to 1975 at the Deutsche Seereederei Rostock. Hence, he was able to travel to Sweden, the Netherlands and Cuba already in his youth.