Learn Finnish: Puhumme terveellisistä tavoista. - We speak about healthy habits.

In Finnish, when you talk about something, the verb puhua requires the topic to be in the elative case. This means the ending must be -sta or -stä. You can think of it as literally meaning 'we speak from healthy habits'.

Why does tapa (habit) become tavoista here?

Two things happen to the base word tapa. First, because we are talking about plural habits, we add the plural marker -i- before the case ending -sta. Second, adding this ending triggers consonant gradation. The strong p in tapa weakens to a v, resulting in tavoista.

How does the adjective terveellinen (healthy) turn into terveellisistä?

Words ending in -nen change that ending to -s- before taking grammatical endings. To make it plural, we add the plural marker -i-, giving us the stem terveellisi-. Finally, because adjectives must match their noun in case, we add the elative ending -stä. It takes the front vowel -stä instead of -sta because the word only contains the neutral vowels e and i.