Complex sentence: Afterwards he cooperated as an unofficial employee for the Ministry for State Security of East Germany between 1962 and 1981 then however he rejected continued service and was himself an Object to Watch of the Stasi from 1984.
Split into simpler sentences: Afterwards he cooperated as an unofficial employee for the Ministry for State Security of East Germany between 1962 and 1981. Then however he refused to continue and was himself an Object of watch of the Stasi from 1984.