Comment by skrebbel

Ahyes forgot about that.

Just to continue the explainer, vowel harmony means that “in the house” is “talossa” but “in the forest” is “metsässä”. Notice that it’s “-ssä” and not “-ssa”. This depends on which vowels are in the root. There’s two groups of vowels and Finnish words helpfully never mix the two in a single (non compound) word.

In other words, “vowel harmony” sounds fancy, but it’s a single

    if root.match(/a|u|o/) then “ssa” else “ssä”