Complete guide to all Finnish noun declensions (almost)

Declining Finnish nouns (and adjectives) is rather simple, despite having 15 cases. All you have to do is to add a case suffix to the “word stem”. Identifying the stem might not be always intuitive, as the nominative usually “disguises” as something else. Here is the excessively detailed diagram to help you identify the stem endings from the appearances of the nouns and adjectives. Let us just focus on the singular forms for simplicity.

Full size: https://imgur.com/a/SHECxZ8

It might look a bit intimidating. Do not trust your intuition. The diagram is unnecessarily detailed in order to list almost all possibilities. Many of them are repeated and you will only need a portion of it and you may soon be able to deduce a similar diagram inside your brain (like I do).

This post assumes the readers are able to gradate the stem consonants whenever necessary and words that require gradation are not to be classified as separate classes. If you do not understand how gradation works, please take a look at this article: https://en.wikipedia.org/wiki/Finnish_consonant_gradation

All the following classes are my personal coinage for the sake of simpler explanation.

Symbol explanations:

The special symbols used in this diagram take reference from the book Colloquial Finnish. They act at phonemes.
#: Vowel copier. It lengthens the previous vowel.
A, O, U: They correspond to a/ä, o/ö, u/y depending on the vowel harmony.
C: An unspecified consonant.
Q: Glottal stop when alone. It geminates (“doubles”) the following consonant (whatever it is, even if the gemination (“doubling”) is normally disallowed).
T: Alternates between t and e (See Sidenote 1 for some extra information)
V: An unspecified vowel.
X: Alternates between # and s. (See Sidenote 2 for extra information)

The following symbols are not in Colloquial Finnish. They signify following footnotes:
$: A dollar sign behind a word denotes unusual vowel harmony.
€: A euro sign behind a word denotes unusual absence of gradation.

Examples and explanations:

Class -VV

This class of nouns/adjectives consists of only 1 syllable and always ends in (real) diphthongs or long vowels. The partitive take -tA and illative takes -h#n.
maa maan maata maahan land, soil, country
syy syyn syytä syyhyn reason
suo suon suota suohon swamp
yö yön yötä yöhön night

Class -VV polysyllabic

This class of nouns/adjectives consists of 2 or more syllables and always ends in (real) diphthongs or long vowels. All the endings and the same as class -VV except for the illative, which takes -seen.
vapaa vapaan vapaata vapaaseen free
Espoo Espoon Espoota Espooseen a Finnish city next to Helsinki where the author lived for about 5 months
suklaa suklaan suklaata suklaaseen chocolate

Class -V (includes but not limited to stem -i and -e)

This is the most common class by far, as most nouns/adjectives end in a vowel. Partitive takes -A.
ilma ilman ilmaa ilmaan air
keho kehon kehoa kehoon body
sänky sängyn sänkyä sänkyyn bed
pöytä pöydän pöytää pöytään table

Post-Finnic loan words and native words with an altered word-final vowel retain their -i stemendings.
muki mukin€ mukia mukiin mug
(from Swedish mugg, from English mug)
grilli grillin grilliä grilliin grill, barbecue
(from Swedish grill)
musiikki musiikin musiikkia musiikkiin music
(through Swedish musik, from Latin musica)
taksvärkki taksvärkin taksvärkkiä taksvärkkiin
(from Swedish dagsverke)
viini viinin viiniä viiniin wine
(through Swedish vin)
tuoli tuolin tuolia tuoliin chair
(from Old Swedish stol)
äiti äidin äitiä äitiin mother
(might not be obvious, but it was from Proto-Germanic *aiþį̄)
koti kodin kotia kotiin home
(Post-Finnic alternation from Finnish words koto or kota)

Most native words (as well as Pre-Finnic loans) with -Ci (when C is neither dental nor h) are -Ce in disguise.
siipi siiven siipeä siipeen wing
Suomi Suomen Suomea Suomeen Finland
joki joen jokea jokeen river
pilvi pilven pilveä pilveen cloud

Comparative ending -mpi is of stem -mpa.
isompi isomman isompaa isompaan bigger
korkeampi korkeamman korkeampaa korkeampaan taller
kumpi kumman kumpaa kumpaan which (not really a comparative ending but it is nonetheless declined this way)

Proper terms with consonant endings takes an extra -i and declines like -i.
Hongkong Hongkongin Hongkongia Hongkongiin
Washington Washingtonin Washingtonia Washingtoniin
Lindholm Lindholmin Lindholmia Lindholmiin (a somewhat popular surname of Scandinavian origin. Also the surname of a very respected, recently deceased Finnish singer. Lepää rauhassa.)

While the vast majority of words with -e ending are actually with stem ending -eQ (see section Class -Q), a small number of words are -e. They are usually either loan words, native words with altered endings, clipping of longer words or just utter irregularity.
itse itsen itseä itseen self
kolme kolmen kolmea kolmeen 3
nalle nallen nallea nalleen teddy bear
Hannele Hannelen Hannelea Hanneleen A female give name

Class -ttoma

The suffix -ton to mean “-less” is inflected as if it is -ttoma (except for partitive). This suffix always gradates the previous stem (ehto + -ton = ehdoton).
tuntematon tuntemattoman tuntematonta tuntemattomaan unknown
ehdoton ehdottoman ehdotonta ehdottomaan unconditional, absolute
loputon loputtoman luputonta luputtomaan endless

Vasen (stem vasempa)

The word vasen is of its own class.
vasen vasemman vasenta vasempaan left

Class -C

Consonant stem ending. An extra -e- may be added in certain cases to comply with the phonotactics. The genitive form is -tA.
kyynel kyynelen kyyneltä kyyneleen tear (see section Class -Q)
askel askelen askelta askeleen step (see section Class -Q)
tytär tyttären tytärtä tyttäreen daughter (cannot decline as -eQ)
jäsen jäsenen jäsentä jäseneen limb (cannot decline as -eQ)
kämmen kämmenen kämmentä kämmeneen palm (cannot decline as -eQ)

Note that the stem kymmen has an irregular -en in the nominative.
kymmenen kymmenen kymmentä kymmeneen 10

Remember, when the letter m is at word-final position or before t, it becomes n. Thus, a number of nouns/adjectives with nominative -n are actually -m when declined.
avoin avoimen avointa avoimeen open
puhelin puhelimen puhelinta puhelimeen telephone
eläin elämen eläintä eläimeen animal
sydän sydämen sydäntä sydämeen heart

The above examples show words with endings with consonants, which -eQ is often (not always) an alternative stem ending (see section Class -Q). In addition to that, Class -C is also found in words with -Ci endings (where C is either dental or h). The stem -t disguises to be -si in the nominative due to historical sound changes.
kieli kielen kieltä kieleen tongue, language
meri meren merta$ mereen sea
veri veren verta$ vereen blood
uni unen unta uneen dream, sleep
lohi lohen lohta loheen salmon
vesi veden vettä veteen water
pursi purren purtta purteen sailboat

Certain words with -Vmi are actually stem -m (some can decline as stem -me as well. Please refer to a dictionary.) Notice that the m changes to n before t.
lumi lumen lunta lumeen snow
toimi toimen tointa toimeen chore, task, job, service

Class -s (nominative: -nen)

A great portion of words which intrinsically end with -nen, as well as those which use it as a diminutive suffix, declines like class -C when C is s.
nainen naisen naista naiseen woman
perhonen perhosen perhosta perhoseen butterfly
hevonen hevosen hevosta hevoseen horse
ihminen ihmisen ihmistä ihmiseen human, person
hetkinen hetkisen hetkistä hetkiseen a short moment

Class -nt

It declines exactly like class -C. Most words of this class are ordinal numbers with -s ending, plus the word tuhat. When inflected, remove the final -s or -t and replace it with -nt-. Whenever the sequence -ntt- appears, remove the first n (see the partitive forms below).
kolmas kolmannen kolmatta kolmanteen 3rd
neljäs neljännen neljättä neljänteen 4th
viides viidennen viidettä viidenteen 5th
tuhat tuhannen tuhatta tuhanteen 1000

Class -ks

It declines exactly like class -C. Words with -Os and intrinsic non-suffix -Us are most certainly of stem -ks. Words with suffix -Us that creates nouns from verbs are also of stem -ks. Native words or recent loans (usually from Greek or Latin) with -is and -es are mostly of stem -ks. Recent loans (usually from Greek or Latin) of -As are also of stem -ks. The sequence -kst- squeezes out the first k.
mainos mainoksen mainosta mainokseen advertisement
päätös päätöksen päätöstä päätökseen decision
kampus kampuksen kampusta kampukseen campus
vastaus vaustauksen vastausta vastaukseen answer (from vastata to answer)
aines aineksen ainesta ainekseen material
diabetes diabeteksen diabetesta diabetekseen diabetes
alias aliaksen aliasta aliakseen alias
One of the possible declensions:
Tuomas Tuomaksen Tuomasta Tuomakseen A male given name

Class -(U)Ut (nominative -(U)Us)

All words with suffix -UUs or -Us which creates nouns from adjectives takes the stem -(U)Ut in singular forms. In fact, -UUs (forming nouns from adjectives) is intrinsically a different suffix from -Us (forming nouns from verbs). It just happens that -UUs looks like -Us when it is appended to certain vowels.
katkeruus katkeruuden katkeruutta katkeruuteen bitterness
ikuisuus ikuisuuden ikuisuutta ikuisuuteen eternity
rakkaus rakkauden rakkautta rakkauteen love
kauneus kauneuden kauneutta kauneuteen beauty
pyhyys pyhyyden pyhyyttä pyhyyteen holiness
terveys terveyden terveyttä terveyteen health

Class -X

This class consists of nouns/adjectives that the ending -s becomes -# when declined (with the exception of the partitive). The vast majority of words ending with -As belong to this class. Old Germanic/Baltic loans with -es or -is are predominantly of this class. As it is often futile to try to know whether a word is a distant loan, memorizing them with brute force is often a more efficient strategy. Note that the illative takes -seen.
There are exactly 2 words ending with -t (actual stem ending: -T) which decline like class -X: kevät, and venät.
Words with suffix -kAs with either means “-y, -ful, -ish” or creates a noun is of stem -kkAX that always gradates the previous stem (e.g. taito + -kas = taidokas).
rakas rakkaan rakasta rakkaaseen dear
ruumis ruumiin ruumista ruumiiseen body, corpse
taidokas taidokkaan taidokasta taidokkaaseen skilful
asiakas asiakkaan asiakasta asiakkaaseen client
äänekäs äänekkään äänekästä äänekkääseen noisy
kevät kevään kevättä kevääseen spring

One of the possible declensions:
Tuomas Tuomaan Tuomasta Tuomaaseen A male given name

Old loans:
rikas rikkaan rikasta rikkaaseen rich
(from Proto-Germanic *rīkijaz)
rengas renkaan rengasta renkaaseen tire, ring
(from Proto-Germanic *hrengaz)
kuningas kuninkaan kuningasta kuninkaaseen king
(from Proto-Germanic *kuningaz)
kaunis kauniin kaunista kauniiseen beautiful
(from Proto-Germanic *skauniz)
kirves kirveen kirvesta kirveeseen axe
(cognate with Lithuanian kirvis)

Class -Q

It is not really a new class. It is just the class -C when the C is a glottal stop. Remember, Q acts as a geminator when it hits a consonant, so -Q + -tA becomes -ttA. The glottal stop is however not written in Finnish orthography. It is typically appended to nouns or adjectives that end with -e in written form, thus the word kone is actually /ˈkoneʔ/ (koneQ) with an unwritten phonemic glottal stop. The only difference between this class and class -C is that the illative is -eseen instead (I could insert an explanation here but it will sidetrack the focus). The Q here was historically an h, so it might explains how it dissolves when it inflects.
vene veneen venettä veneeseen boat
raide raiteen raidetta raiteeseen railroad track
osoite osoitteen osoitetta osoiteeseen address

Certain Class -C words can be declined as -eQ.
kyynel(e) kyyneleen kyynelettä kyyneleeseen tear
askel(e) askeleen askeletta askeleeseen step

Class -T

The most singular nouns with -t turns the final consonant to -e- when declined, except the partitive (thus -T, see section Symbol explanations).
olut oluen olutta olueen beer
ohut ohuen ohutta ohueen thin
kevyt kevyen kevyttä kevyeen light

Class -Ut particle (stem -ee)

Verbs can take -nUt (as well as -sUt, -lUt or -rUt in verbs with endings -stA, -llA and -rrA) to form past active particles. The -Ut becomes -ee- in all inflected cases but partitive. Illative takes -seen.
luonut luoneen luonutta luoneeseen (that have) made
päässyt päässeen päässyttä päässeeseen ( that have) reached
kuollut kuolleen kuollutta kuolleeseen dead, (that have) died
purrut purreen purrutta purreeseen (that have) bitten

Before you go…

This article does not aim to tell the readers to memorize all the declension patterns. This is more like a torch or a road map for beginners in learning the Finnish language. Once you know what is happening, your brain will build a similar road map like what I have done here. As a grammar nerd who does not even speak Finnish, I am happy to share this piece of knowledge with you. By no means I claim this guideline to be 100% fool-proof, as minor irregularities do happen, but I am hopeful that the article is helpful to you in declining the vast majority of Finnish nouns and adjectives.

Sidenotes

This section is merely for some historical linguistic backgrounds. It is not necessary for beginners.

1. The T there is not really alternating between t and e. It is actually either a t or a phantom null consonant (which was historically a weak d by the way). The e there was just to link the consonants behind the weak d (which has now become null) to make it phonotactically legal.
2. Historically there was no X. It was just a weakened s. The weak grade of s appears to have been h. The genitive of rengas was probably *renkahen. What happened is that the e became # and the h disappeared, so it is now renkaan. This s-to-h change still lingers in certain words: mies (miehen miestä mieheen).

Reference

Abondolo, D. M., Björklund, H., Multanen, E., & Valijärvi, R. (2015). Colloquial Finnish: The complete course for beginners. Place of publication not identified: Routledge.