Complex sentence: Not until 1942 was Hälsingland granted its third city, Bollnäs, which was to become the last city (in Hälsingland) as City status in Sweden was abolished in 1971.
Split into simpler sentences: Not until 1942 was Hälsingland granted its third city, that of Bollnäs. It was to become the last city (in Hälsingland) as City status in Sweden was abolished in 1971.