Duolingo is a US app, geared towards teaching native English speakers a host of foreign languages. Articles are an integral part of most other languages, including English. At least you don't have to deal with gendered nouns and their respective articles, like un/une and le/la as you did when you studied French.
To everyone who says Helsinki isn't a small city, it is definitely extremely small for a capital city. To put into perspective, Madrid, which is a mediumish capital compared to other European capitals, has about five times as many inhabitants in just the city (not counting surrounding areas).