Learn Hardcore Finnish: Kun en löydä avainta, etsin sitä repusta. - When I can’t find the key, I look for it in the backpack.

Kun introduces a temporal clause meaning “when” or “whenever,” especially for habitual or repeated actions. Here it shows “whenever I can’t find the key…”. Jos means “if” and expresses a more hypothetical condition (“if I happen not to find the key…”). Using Jos would make it less about a regular occurrence and more about a single possible scenario.

Why is avain in the partitive form avainta instead of the nominative avain?

In Finnish, direct objects take the partitive case after negative verbs and when the action is incomplete or ongoing. Because the verb is negative (en löydä), avain becomes avainta. It also underlines that you haven’t found the key yet.

Why is there sitä in the sentence, and why is it in the partitive form?

Sitä is the partitive form of the demonstrative pronoun se (“it”). You use it to refer back to avainta (“the key”) instead of repeating avainta. The partitive (sitä) is used because etsiä (“to search for”) typically takes a partitive object when the search is not yet completed.

Why is repusta used instead of reppu or reppussa, and what does -sta mean?

Repusta is the elative case (stem reppu + -sta) meaning “out of/from the bag.” Finnish uses elative to express searching “from inside” something (etsiä + jostakin). Reppussa would be the inessive (“in the bag”), but the idiomatic collocation with etsiä is elative repusta.

Why is there no subject pronoun like minä (“I”) in either clause?

Finnish is a pro-drop language: the subject pronoun is usually omitted because the verb ending already shows person and number. Here en löydä and etsin both signal first‐person singular, so minä is unnecessary.

Could you omit sitä and just say Kun en löydä avainta, etsin repusta?

Yes. If the context makes it clear what you’re searching for, the pronoun sitä can be dropped:
Kun en löydä avainta, etsin repusta.
This is perfectly natural.

What’s the difference between sitä and sen as the pronoun “it”?

Sitä is partitive; sen is genitive/accusative. You use the partitive sitä with etsiä when the action is not completed. If you were talking about a completed, telic action (“I found it”), you could use sen:
Löysin sen avaimen. (“I found that key.”)

How would you say “Whenever I can’t find the keys, I look in my bag” (plural) in Finnish?

Use the plural partitive avaimia and add aina for “whenever”:
Aina kun en löydä avaimia, etsin repusta.

If you wanted to express cause and effect (“I can’t find the key, so I look in my bag”), how would you connect the clauses?

You can replace Kun with a main‐clause conjunction like joten or niin plus comma:
En löydä avainta, joten etsin sitä repusta.
This shifts from a temporal “when/whenever” to a causal “so.”