Rule

Built from the present tense, but you add an "i" in between the stem and the personal ending. It's easiest to do this with the minä form, or at least don't use third person since some weird things can happen there.

Endings

o, ö, u, y

• These are the easiest ones, you just add the i afterwards:

• sanon -> sanoin
• säilön -> säilöin
• puhun -> puhuin

a, ä, e, i

• Replace the vowel with "i"

• odotan -> odotin

a, but there is another "a" earlier in the word

• These ones are a bit weird - if the word has an "a" in the beginning and also ends in an "a", then you replace the last "a" with "o", and then treat it as an -o ending.
• Only for two-syllable words

• alan -> aloin
• laulan -> lauloin

Double vowel

This goes to the second vowel + i:

uo -> oi
yö -> öi
ie -> ei

juon -> join
syön -> söin
vien -> vein

Except for "käyda", which is a special case.

"si" words

• All the verbs in the fourth verb type (herään, haluan, vastaan)
  • These get "si" added - heräsin, halusin, vastasin

• herään -> heräsin
• haluan -> halusin
• vastaan -> vastasin

• There are a few type 1 verbs that also get this if they sound kind of like type four verbs. Here is a list of them:
  kieltää, kääntää, rakentaa, työntää, siirtää, piirtää
  and
  huutaa, löytää, pyytää, tuntea, tietää, taitaa

Exceptions

The only irregular verb in Imperfektii is "käydä", which goes to "minä kävin"

!Imperfekti Suomen Mestari 2, p.1
!Imperfekti Suomen Mestari 2, p.2
!Imperfekti Suomen Mestari 2, p.2