Learn Finnish: Olin uimassa, kun ilta pimeni. - I was swimming when the evening got dark.

It comes from the Verb Type 6 word pimetä (to get dark). In Type 6 verbs, the stem is formed by dropping the -tä and adding -ne-, giving us the stem pimene-. To make it past tense for 'it got dark' (third person singular), the final -e drops before the past tense marker -i, resulting in pimeni.

Does pimetä come from the adjective pimeä?

Yes! Many Type 6 verbs express a change of state, meaning 'to become [adjective]'. Pimetä comes from pimeä (dark) and means 'to become dark'. Other common examples you might know are kylmetä (to become cold) and lämmetä (to become warm).

Why use olin uimassa instead of just the past tense uin (I swam)?

Using the verb olla (to be) plus the -massa or -mässä form of a verb emphasizes an ongoing action. Olin uimassa translates to 'I was in the middle of swimming'. This structure is very common when setting the background scene for an interrupting event, like the evening getting dark.

How would I say the evening didn't get dark?