CHANGES OF SOUNDS.
All Finnish words consist of a root to which certain suffixes have been attached, but the addition of these latter often causes certain changes in the final vowel and consonants of the original root. In order to inflect words correctly it is necessary to know these changes.
A. Changes of Vowels.
I. The Long Vowels and Diphthongs.
3. The long vowels are always shortened when followed by i, that is to say, the vowel is written once and not twice. Thus maa, earth, which takes an i in all the cases in the plural except the nominative, forms maita, maitten, maiksi, etc., not maaita; puu, tree, puita, puiden, puiksi.
4. The diphthongs with the accent on the second vowel, uo, yö, ie, when followed by i, reject the first vowel, and thus form a new diphthong—työ, work, becomes in the plural töistä, töissä, töiksi, etc.; vien, I lead, vein (vie-in), I led; luo, he creates, loisi (luo-isi), he would create.
5. Diphthongs ending in i reject this i if another i follows, uin, I swim, uin (for ui-in), I swam.
In the root käy, to go, y is changed into v before a vowel—e.g. käy-in becomes kävin.