The strong vowel stem of verbtype 1 is used in:

• the infinitive (e.g. lukea)
• the present tense third person singular and plural (e.g. lukee, lukevat)
• the imperfect tense third person singular and plural (e.g. luki, lukivat)
• the conditional, all persons (e.g. lukisin, lukisit, lukisi, lukisimme, lukisitte, lukisivat)
• the plural imperative (e.g. lukekaa) – also in the active jussiivi forms (e.g. lukekoon, lukekoot).
• the active present participle (e.g. lukeva)
• the active past participle (e.g. lukenut) – also used in the negative imperfect (e.g. en lukenut)
• the potential, all persons (e.g. lukenen, lukenet, lukenee, lukenemme, lukenette, lukenevat)
• the MA-infinitive, third infinitive (e.g. lukemassa, lukemaan, lukemasta, lukematta)
• the MINEN-form, fourth infinitive (e.g. lukeminen, lukemista, lukemisesta, lukemisen)

The final vowel of the vowel stem undergoes many changes when we’re conjugating verbs. For example, for verbs stems ending in an -e, the -e will be replaced with an -i- in the past tense and the conditional (e.g. luen : luin : lukisin). The imperfect tense is the form where the most variation happens (e.g. laulan : lauloin; tietää : tiesin).

The weak stem of verbtype 1 is used in:

• the present tense first and second person singular and plural (e.g. luen, luet, luemme, luette)
• the imperfect tense first and second person singular and plural (e.g. luin, luit, luimme, luitte)
• the singular imperative (e.g. lue)
• all the passive forms: e.g. present passive (e.g. luetaan), past passive (e.g. luettiin), passive conditional (e.g. luettaisiin)
• the passive present participle (e.g. luettava)
• the passive past participle (e.g. luettu) – also used in the negative past passive (e.g. ei luettu)

Doesn’t exist!

Verbtype 2 only has one stem: the vowel stem

The vowel stem of verbs like syödä, juoda and viedä will undergo changes in the imperfect and the conditional. The diphthong at the end of the stem will shorten to accommodate for the –i– that is added (e.g. syön: sö-i-n, sö-isi-n).

Only the verbs tehdä and nähdä have a consonant stem. These two verbs are also the only ones in verbtype 2 with consonant gradation in the vowel stem.

The vowel stem of verbtype 3 is always strong and used in:

• the present tense, all persons (e.g. ompelen, ompelet, ompelee, ompelemme, ompelette, ompelevat)
• the imperfect tense, all persons (e.g. ompelin, ompelit, ompeli, ompelimme, ompelitte, ompelivat)
• the conditional, all persons (e.g. ompelisin, ompelisit, ompelisi, ompelisimme, ompelisitte, ompelisivat)
• the active present participle (e.g. ompeleva)
• the singular imperative (e.g. ompele)
• the MA-infinitive, third infinitive (e.g. ompelemassa, ompelemasta, ompelemaan, ompelematta)
• the MINEN-form, fourth infinitive (e.g. ompeleminen, ompelemista, ompelemisesta, ompelemisen)

For these verbs, the vowel stem’s final –e will be replaced by an –i– in the imperfect tense and the conditional. Only verbs ending in -lla will have consonant gradation.

The consonant stem of verbtype 3 is always weak and used in:

• all the passive forms: e.g. present passive (e.g. ommellaan), past passive (e.g. ommeltiin), passive conditional (e.g. ommeltaisiin)
• the active past participle (e.g. ommel-lut) – also used in the negative imperfect (e.g. en ommellut)
• the passive present participle (e.g. ommel-tava)
• the passive past participle (e.g. ommel-tu), – also used in the negative past passive (e.g. ei ommeltu)
• the potential, all persons (e.g. ommel-len, ommel-let, ommel-lee, ommel-lemme, ommel-lette, ommel-levat)
• the plural imperative (e.g. ommel-kaa) – also in the active jussiivi forms (e.g. ommelkoon, ommelkoot).

The vowel stem of verbtype 4 is always strong and used in:

• the present tense, all persons (e.g. tapaan, tapaat, tapaa, tapaamme, tapaatte, tapaavat)
• the imperfect tense, all persons (e.g. tapasin, tapasit, tapasi, tapasimme, tapasitte, tapasivat)
• the conditional, all persons (e.g. tapaisin, tapaisit, tapaisi, tapaisimme, tapaisitte, tapaisivat)
• the singular imperative (e.g. tapaa)
• the active present participle (e.g. tapaava)
• the MA-infinitive, third infinitive (e.g. tapaamassa, tapaamasta, tapaamaan, tapaamatta)
• the MINEN-form, fourth infinitive (e.g. tapaaminen, tapaamista, tapaamisesta, tapaamisen)

For these verbs, the long vowel at the end of the vowel stem will be shortened in the imperfect tense and the conditional (e.g. tapaan : tapasin : tapaisin).

The consonant stem of verbtype 4 is always weak and used in:

• the plural imperative (e.g. tavat-kaa) – also in the active jussiivi forms (e.g. tavat-koon, tavat-koot).
• the present passive (e.g. tavat-aan – alternatively tava-taan*)
• the past passive (e.g. tavat-tiin)
• the passive conditional (e.g. tavat-taisiin)
• the passive present participle (e.g. tavat-tava)
• the passive past participle (e.g. tavat-tu) – also used in the negative past passive (e.g. ei tavattu)
• the active past participle (e.g. tavan-nut – alternatively tava-nnut**) – also used in the negative imperfect (e.g. en tavannut)
• the potential, all persons (tavan-nen – alternatively tava-nnen**, tavannet, tavannee, tavannemme, tavannette, tavannevat) 

* Advanced: tava-taan vs tavat-aan

With verbtypes 4, 5 and 6, the only forms with a completely unambiguous consonant stem are the imperative conjugations (e.g. tavat-kaa, tavat-kaamme, tavat-koon, tavat-koot). Apart from those forms, there are multiple approaches to deciding where the stem ends, and the conjugation markers start. This is apparent in the passive and potential conjugation. Should we, for example, analyze the present passive tavataan as:

Read more about this here and here. The fact that both of these sources are in Finnish and written by linguists, should show you that this is a very theoretical dilemma.

** Advanced: tava-nnut vs tavan-nut

In the active past participle, the consonant stem’s final –t (e.g. tavat-) has historically assimilated into an –n (“tavat-nut” → tavan-nut). Many current sources, however, prefer to see the -nnut as a whole. This is easier for language learners, for whom the origin of the -nn- is of no importance. This interpretation does raise the question concerning the stem used for this form: is it the historical consonant stem (tavat-), or is it the new stem I addressed above (tava-). If the latter, we could say we use this new stem for both tava-nnut and tava-taan.

VISK marks these as, for example, tava=n=nut, thus indicating that we could approach this as both tavan-nut and tava-nnut.

The vowel stem of verbtype 5 is used in:

• the present tense, all persons (e.g. tarvitsen, tarvitset, tarvitsee, tarvitsemme, tarvitsette, tarvitsevat)
• the imperfect tense, all persons (e.g. tarvitsin, tarvitsit, tarvitsi, tarvitsimme, tarvitsitte, tarvitsivat)
• the conditional, all persons (e.g. tarvitsisin, tarvitsisit, tarvitsisi, tarvitsisimme, tarvitsisitte, tarvitsisivat)
• the singular imperative (e.g. tarvitse)
• the active present participle (e.g. tarvitseva)
• the MA-infinitive, third infinitive (e.g. tarvitsemassa, tarvitsemasta, tarvitsemaan, tarvitsematta)
• the MINEN-form, fourth infinitive (e.g. tarvitseminen, tarvitsemista, tarvitsemisesta, tarvitsemisen)

For these verbs, the vowel stem’s final –e will be replaced by an –i– in the imperfect tense and the conditional.

The consonant stem of verbtype 5 is used in:

• the plural imperative (e.g. tarvitkaa) – also in the active jussiivi forms (e.g. tarvit-koon, tarvit-koot).
• the present passive (e.g. tarvitaan)
• the past passive (e.g. tarvittiin)
• the passive conditional (e.g. tarvittaisiin)
• the passive present participle (e.g. tarvittava)
• the passive past participle (e.g. tarvittu) – also used in the negative past passive (e.g. ei tarvittu)
• the active past participle (e.g. tarvinnut) – also used in the negative imperfect (e.g. en tarvinnut)
• the potential, all persons (tarvinnen, tarvinnet, tarvinnee, tarvinnemme, tarvinnette, tarvinnevat)

* Just like with verbtype 4, we have the problem of deciding where the stem ends, and where the conjugation markers start in the passive and potential conjugation. Should we analyze the present passive as:

• Option 1: punnit-aan (consonant stem + -aan).
• Option 2: punnita-an (infinitive + -an)
• Option 3: punni-taan (a completely new stem + -taan), which would making this a verbtype with three stems: the traditional consonant stem punnit-, the traditional vowel stem punnitse- and the additional vowel stem punni-).

** We have the same problem with the NUT-participle and the potential conjugation: for punnit-a, the participle’s -nut and the potential’s -ne markers have caused the -t- at the end of the consonant stem to assimilate into an -n-: the historical/theoretical punnit-nut has become punnin-nut, and punnit-nen has become punnin-nen.

VISK marks these as, for example, punni=n=nut, thus indicating that we could approach this as both punnin-nut and punni-nnut.

The vowel stem of verbtype 6 is strong and used in:

• the present tense, all persons (e.g. lämpenen, lämpenet, lämpenee, lämpenemme, lämpenette, lämpenevät)
• the imperfect tense, all persons (e.g. lämpenin, lämpenit, lämpeni, lämpenimme, lämpenitte, lämpenivät)
• the conditional, all persons (e.g. lämpenisin, lämpenisit, lämpenisi, lämpenisimme, lämpenisitte, lämpenisivät)
• the singular imperative (e.g. lämpene)
• the active present participle (e.g. lämpenevä)
• the MA-infinitive, third infinitive (e.g. lämpenemässä, lämpenemästä, lämpenemään, lämpenemättä)
• the MINEN-form, fourth infinitive (e.g. lämpeneminen, lämpenemistä, lämpenemisestä, lämpenemisen)

The final vowel of the vowel stem undergoes some changes when we’re conjugating verbs. The vowel stem’s final -e will be replaced with an -i- in the past tense and the conditional (e.g. lämpenen : lämpenin : lämpenisin).

The consonant stem of verbtype 6 is weak and used in:

• the plural imperative (e.g. lämmetkää) – also in the active jussiivi forms (e.g. lämmetköön, lämmetkööt).
• the present passive (e.g. lämmetään)
• the past passive (e.g. lämmettiin)
• the passive conditional (e.g. lämmettäisiin)
• he passive present participle (e.g. lämmettävä)
• the passive past participle (e.g. lämmetty)
• the active past participle (e.g. lämmennyt)
• the potential, all persons (lämmennen,  lämmennet, lämmennee, lämmennemme, lämmennette, lämmennevät)

Once again, we have some unclarity when deciding what the best approach is for marking the stem in this verbtype. Look at the full explanation in the box labeled “verbtype 4 weak consonant stem”. VISK likes to mark these as vanhe=t=aan, vanhe=t=tiin, vanhe=t=tu, vanhe=n=nen, etc.