Two of the first conjunctions you meet in Finnish, kun and koska, both land on English "when," "since," or "because" — and English is no help in keeping them apart. The rule is clean: kun marks time ("when, as, while"), and koska marks cause ("because"). This page is the decision rule and the traps; it does not re-teach the whole conjunction system, so follow the links for the full inventory.
The one decision
Ask one question: am I saying when something happens, or why it happens?
- Setting the time frame of an event → kun (when, as, while)
- Giving the reason for something → koska (because)
The classic minimal pair uses almost the same words, and only the conjunction changes the meaning:
Kun sataa, jään kotiin.
When it rains, I stay home. (time — kun sets the condition/moment)
Jään kotiin, koska sataa.
I stay home because it rains. (cause — koska answers 'why')
Notice that both sentences are grammatically complete and both are natural — the speaker simply chooses to frame the rain as a time ("whenever it rains") or as a reason ("the reason I'm staying in"). That framing is your whole decision.
Kun olin nuori, asuin Oulussa.
When I was young, I lived in Oulu. (a time frame in the past)
En tullut, koska olin sairas.
I didn't come because I was sick. (the reason I didn't come)
kun = the time frame
kun answers when / as / while. It can point to a single moment, a stretch of time, or a repeated situation, and it works with any tense.
Soita minulle, kun pääset perille.
Call me when you get there. (a future moment)
Kun katselin televisiota, puhelin soi.
While I was watching TV, the phone rang. (two events, one framing the other)
Kun tulin kotiin, kaikki jo nukkuivat.
When I got home, everyone was already asleep.
koska = the reason
koska answers why. It introduces the cause, and the clause it starts can come either before or after the main clause — the meaning is the same, only the emphasis shifts.
Emme lähteneet, koska satoi.
We didn't leave because it was raining. (reason follows)
Koska olin väsynyt, menin aikaisin nukkumaan.
Because I was tired, I went to bed early. (reason first)
Miksi olet iloinen? — Koska sain työpaikan!
Why are you happy? — Because I got the job!
That last exchange is the surest test: koska is the natural answer to a miksi? ("why?") question. kun can never answer miksi? — it answers milloin? ("when?").
Why English speakers get this wrong
The trap is English "when" and "since," which quietly do double duty. "When you're finished, call me" is temporal → kun, but a sentence like "Since you're already here, help me" is causal → koska, not kun. English "since" is the real ambush: in "since Monday" it is temporal (and translated with a case, not a conjunction), while in "since you ask" it is a reason → koska. Do not let the single English word pull you toward kun when you actually mean because.
Koska olet jo täällä, voit auttaa minua.
Since (= because) you're already here, you can help me.
The overlap you can safely ignore
There is one honest gray zone. kun with a past-tense state can shade into a reason-flavored "now that / seeing as": "Kun kerran lupasit, sinun täytyy tulla" ("Since you did promise, you have to come"). Here the time frame and the reason blur together, and Finns do use kun for it. This is a stylistic nuance, not a licence to swap the two words — when the sentence is purely "X because Y," keep koska. For the finer points of this overlap and the rest of the conjunction family, see koska vs kun in the conjunctions section.
Do not confuse kun with its near-homophone kuin ("than / as," used in comparisons): nopeampi kuin sinä ("faster than you"). They sound almost identical but do completely different jobs — that pair has its own page, kuin vs kun.
Common Mistakes
❌ Jään kotiin, kun olen sairas.
Incorrect if you mean 'because I'm sick' — a reason needs koska.
✅ Jään kotiin, koska olen sairas.
I'm staying home because I'm sick.
❌ Koska olin nuori, asuin Oulussa.
Incorrect for 'when I was young' — a time frame needs kun.
✅ Kun olin nuori, asuin Oulussa.
When I was young, I lived in Oulu.
❌ Miksi et syönyt? — Kun en ollut nälkäinen.
Incorrect — the answer to miksi? (why?) is koska.
✅ Miksi et syönyt? — Koska en ollut nälkäinen.
Why didn't you eat? — Because I wasn't hungry.
❌ Kun sinä kysyt, kerron totuuden.
Incorrect if you mean 'since you ask' (a reason) — use koska.
✅ Koska sinä kysyt, kerron totuuden.
Since (= because) you ask, I'll tell the truth.
Key Takeaways
- kun = time ("when / as / while / once"); it answers milloin?
- koska = cause ("because"); it answers miksi? and can front or follow the main clause.
- English "when" and especially "since" hide the split — decide whether you mean a moment on the timeline or a reason.
- Standard written Finnish never uses kun for a pure cause, even though clipped ku does that job (informal) in speech.
- Keep kun apart from kuin ("than"), a different word that only sounds alike.
Now practice Finnish
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Start learning Finnish→Related Topics
- 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.
- kuin vs kunB2 — The comparison word kuin ('than, as') versus the temporal kun ('when') — a one-letter difference that spoken Finnish hides but writing demands.
- että-Clauses in DepthB1 — The finite että complement clause in depth: which verbs take it, why Finnish keeps the original tense, and the se että 'the fact that' pattern.
- Conjunctions: Coordinating and SubordinatingA1 — Finnish conjunctions in two families — coordinating (ja, tai, mutta) and subordinating (että, koska, kun, jos) — with the comma rule and one-to-many mappings.