Original: Turku quickly became the most important city in Finland and retained that status for hundreds of years until Finland became part of the Russian Empire in 1809 and the capital of the Grand Duchy of Finland was transferred to Helsinki in 1812. After that Turku continued to be the most populous city in Finland until the end of the 1840s.
Simplified: Turku used to be the largest city and capital of Finland, until 1812, when it was moved to Helsinki.