Learn Finnish: Kolmannes kansalaisista asuu tässä taajamassa. - A third of the citizens live in this urban area.

Both words mean "a third." Kolmasosa (third part) is the everyday compound word you likely learned earlier. Kolmannes is the more formal version using the -nes suffix. You form it by taking the inflectional stem of the ordinal number kolmas (third), which is kolman-, and adding -nes.

Why is the verb asuu in the singular when referring to many citizens?

The grammatical subject of this sentence is the fraction kolmannes, which is a singular noun. In Finnish, the verb always agrees with the grammatical subject, so it takes the third-person singular form asuu, even though it refers to a plural group of people.

Why does "of the citizens" translate into the elative case kansalaisista?

When expressing "out of a larger group," Finnish uses the elative case (-sta/-stä). After a fraction like kolmannes or a quantity word, the group being divided is put into the elative to show what the fraction is drawn from.

What kind of place is a taajama compared to a kaupunki (city)?

While kaupunki simply means city or town, taajama is a specific geographic and statistical term for a "built-up area" or "urban cluster." In Finland, it refers to any densely populated area with at least 200 inhabitants, regardless of whether it is officially part of a city or a rural municipality.