Learn Finnish: Kolmasosa vastaajista on naisia. - A third of the respondents are women.

Fractions are typically built by combining the ordinal number (like kolmas, meaning "third") with the word osa (meaning "part"). So, kolmasosa literally means "third part". Other examples follow the same pattern, like neljäsosa (a fourth) and viidesosa (a fifth).

Why is vastaajista in the elative (-sta) case?

In English, we say "a third of the respondents". In Finnish, when you express a fraction, percentage, or amount coming out of a specific group, you put that group into the elative case (-sta/-stä). You can think of it as meaning "a third out of the respondents".

Why does the sentence use the singular verb on (is) when the English translation says "are"?

In Finnish grammar, the grammatical subject of this sentence is the fraction kolmasosa (a third). Because kolmasosa is a singular word, the verb must also be singular (on). English focuses on the plural "respondents" to choose the verb, but Finnish strictly agrees with the singular fraction.

Why is naisia (women) in the partitive plural?

Here, naisia describes what the subject (kolmasosa) consists of. When describing a subject as belonging to an undefined, plural mass or category (some women, not all specific women), Finnish uses the partitive plural.