II. Short Vowels.
The simple vowels o, ö, u, y are invariable, but a, ä, e, i are subject to certain changes in the last syllable of a root.
A. Ä.
When the vowels A and Ä at the end of a root are followed by the i characteristic of the imperfect tense, or plural, they undergo the following changes:—
6. (1) Ä is always rejected in disyllables—e.g. heitän, I throw, heitin, I threw.
7. (2) A at the end of disyllabic roots is rejected before i, if the vowel of the first syllable is o, u, uu, ou, uo, oi, or ui: but is changed into o if that vowel is a, e, i, aa, ii, ai, au, ei, eu, ie, or iu. Thus ottavat, they take, past ottivat (for ottaivat); nuora, a cord, nuorilla, with cords; tupa, a hut, tuvissa, in huts. But on the other hand, kala, a fish, kaloiksi; annan, I give, annoin, I gave; kannan, I carry, kannoin, I carried.[5]
Disyllabic verbs, where the final a is preceded by t, can change it into o, but generally reject it, ahtoi or ahti: kaartoi or kaarti.
8. Derivatives of verbs in ma, ja, va, always reject the a: sanova, sanovia; ottaja, ottajia; puhuva, puhuvia.
9. (3) Polysyllabic verbs always reject a and ä in the imperfect. Odotan, I wait, odotin, I waited, etc. The rejection also takes place in polysyllabic substantives ending in -mpa, -mpä, and derivatives in va, vä, sa, sä, and those where h or any vowel but i precedes the final a, e.g. vanhempa, vanhemmille; sanova, sanovina; vieraha, vierahille; kapea, kapeita. But should the final a be preceded by two consonants, or the penultimate syllable contain the vowel i, a and ä become respectively o and ö; asia, a thing, asioissa; karitsa, a lamb, karitsoita; kynttilä, a candle, kynttilöitä, candles. But the words isäntä, emäntä always reject i, isännille, emännille.
10. (4) In cases which come under none of these heads, e.g. such a word as peruna, potatoe, a, ä can be either dropped or changed to o, ö, perunia or perunoita, partitive case plural.