These two words differ by a single letter and by nothing at all in casual speech, yet they belong to completely separate corners of the grammar. kuin is the word of comparison — "than, as." kun is the word of time — "when." Confusing them is one of the most visible mistakes a learner can make in writing, precisely because Finns themselves say both as a mumbled ku and rely on context. In careful writing there is no context to lean on: you have to know which one you mean.
kuin — comparison ("than, as")
kuin links the two sides of a comparison. It follows a comparative adjective or adverb to mean "than," and it pairs with yhtä or niin to mean "as … as."
Helsinki on paljon suurempi kuin Tampere.
Helsinki is much bigger than Tampere. (comparative + kuin = 'than')
Tänään on kylmempää kuin eilen.
It's colder today than yesterday.
Hän juoksee nopeammin kuin minä.
He runs faster than I do. (comparative adverb + kuin)
Kahvi ei ollut yhtä hyvää kuin viime kerralla.
The coffee wasn't as good as last time. (yhtä ... kuin = 'as ... as')
After kuin the second party usually stands in the nominative — kuin minä, kuin sinä, kuin Tampere — because it mirrors the subject of the comparison. Finnish also offers a slicker option: drop kuin entirely and put the compared noun in the partitive before the comparative. parempi kuin minä and minua parempi mean the same "better than me."
Sisareni on pari vuotta minua vanhempi.
My sister is a couple of years older than me. (partitive minua, no kuin)
For the full mechanics of these two "than" strategies, see than-comparison and how the comparative degree is built on comparatives.
kun — time ("when, as, while")
kun is a subordinating conjunction of time and circumstance: "when," "as," "while," "now that." It heads a clause that sets the temporal frame for the main clause. It is never a comparison word.
Kun olin nuori, asuimme meren rannalla.
When I was young, we lived by the sea. (temporal kun)
Kun saavuin asemalle, juna oli jo lähtenyt.
When I arrived at the station, the train had already left.
Soitan sinulle heti, kun pääsen kotiin.
I'll call you the moment I get home. (kun + present for a future 'when')
Notice the last example: for a future "when," Finnish keeps kun with the present tense (kun pääsen), where English also uses the present ("when I get home"). kun also shades into "since / now that" when it gives a reason grounded in a situation:
Nyt kun sinä olet täällä, voimme aloittaa.
Now that you're here, we can start.
That "now that / since" sense sits right next to koska ("because"). The line between temporal kun and causal koska is its own decision — see koska vs kun for where circumstance ends and cause begins.
The minimal pair
Hän on parempi kuin minä.
He is better than me. (comparison → kuin)
Hän oli iloinen, kun voitti.
He was happy when he won. (time → kun)
Read them aloud and, in ordinary speech, both middle words come out as ku — which is exactly why the distinction has to live in your head, not your ear.
Don't confuse kuin with ennen kuin
There is one fixed phrase where kuin appears without any comparative in sight: ennen kuin ("before"). Here kuin is welded to ennen to form a temporal conjunction. It looks temporal, but it is spelled with the comparison word — a spelling learners routinely get wrong by writing ennen kun.
Soita minulle ennen kuin lähdet.
Call me before you leave. (fixed phrase: ennen kuin, not ennen kun)
Ehdimme kotiin ennen kuin alkoi sataa.
We made it home before it started to rain.
See ennen kuin for the full construction. The takeaway: "before" is ennen kuin with kuin, even though the meaning is about time.
Why English speakers get this wrong
English keeps "than" and "when" as visibly different words, so an English speaker does not mean to swap them — the error comes entirely from Finnish itself. In spoken Finnish both collapse to ku(n), and much of the input a learner hears offers no clue which word underlies the sound. On top of that, kun looks like the more "basic" word, so it gets over-generalised into comparisons where kuin belongs. The fix is a hard rule that ignores the pronunciation: comparison → kuin (with the i), time → kun (no i), "before" → ennen kuin.
Common Mistakes
❌ Hän on vanhempi kun minä.
Incorrect — this is a comparison, so it needs kuin: vanhempi kuin minä.
✅ Hän on vanhempi kuin minä.
He is older than me.
❌ Kuin olin lapsi, pelkäsin pimeää.
Incorrect — this is about time, so it needs kun: kun olin lapsi.
✅ Kun olin lapsi, pelkäsin pimeää.
When I was a child, I was afraid of the dark.
❌ Talo on kaksi kertaa isompi kun ennen.
Incorrect — comparison again: isompi kuin ennen.
✅ Talo on kaksi kertaa isompi kuin ennen.
The house is twice as big as before.
❌ Pese kädet ennen kun syöt.
Incorrect — the fixed phrase 'before' is ennen kuin, with kuin.
✅ Pese kädet ennen kuin syöt.
Wash your hands before you eat.
❌ Kahvi ei ole yhtä hyvää kun ennen.
Incorrect — 'as ... as' uses kuin: yhtä hyvää kuin ennen.
✅ Kahvi ei ole yhtä hyvää kuin ennen.
The coffee isn't as good as it used to be.
Key Takeaways
- kuin (with the i) = comparison: "than" after a comparative (isompi kuin), and "as … as" with yhtä … kuin.
- kun (no i) = time and circumstance: "when, as, while, now that" (kun tulin, nyt kun).
- After kuin the compared party is usually nominative (kuin minä); or drop kuin and use the partitive (minua parempi).
- "Before" is ennen kuin — spelled with kuin, even though it is about time.
- Spoken Finnish reduces both to ku, so trust the rule, not the sound; the i is the whole difference.
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Start learning Finnish→Related Topics
- Saying 'than': kuin and the Partitive of ComparisonB1 — Finnish says 'than' two ways: kuin + nominative (pidempi kuin minä) or the partitive of comparison with no kuin (minua pidempi).
- The Comparative -mpi: 'more'A2 — Finnish builds the comparative with the suffix -mpi on the adjective stem: iso → isompi, vanha → vanhempi, kaunis → kauniimpi, hyvä → parempi.
- Temporal Clauses: ennen kuin, sen jälkeen kun, kunnesB1 — The multi-word temporal connectives — ennen kuin (before), sen jälkeen kun (after), heti kun (as soon as), kunnes (until) — and the kuin/kun trap inside them.
- Cause and Time: koska, kun, kunnesA2 — koska gives the reason ('because'), kun marks time ('when/as'), and kunnes marks the endpoint ('until') — plus the deadly kun/kuin trap, one vowel apart.
- Getting 'than' WrongB1 — The two ways Finnish says 'than' — kuin + nominative, or the partitive without kuin — plus the kuin/kun mix-up and the 'more + adjective' calque, drilled as wrong→right pairs.