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Vowel harmony is a very typical phenomenona in Finno-Ugric languages.
There cannot be front vowels (y, ö, ä) and back vowels (u, o, a) in a same word.
This rule is important when using different suffixes. There is usually two versions (allomorphs) of one suffix, eg. -nut /-nyt, -ko/kö , -ssa/-ssä, la/lä.
The rule of thumb: if there's already vowels with dots in the word, the suffix should have dots, too. (Think y as ü.)
I and e are neutral, and they can mix. When they are alone, they are usually counted as front vowels.
Inessive: Minä asun Turussa, but Hän asuu Jyväskylässä
Ko-question Tuletko mukaan? but Lähdetkö pois?
Nut-participle: En ole nukkunut hyvin but, Olen syönyt paljon
Other suffixes: Hän on ammatiltaan taitelija, but Minä olen piirtäjä