Learn Finnish: Etsimämme laite on se, joka on rikki. - The device we are looking for is the one that is broken.

In Finnish, the main clause needs a grammatical complement. You cannot just attach a relative clause starting with joka directly to the verb on here. The demonstrative pronoun se acts as the required complement ("it" or "the one"), creating the pseudo-cleft structure se, joka ("the one that"). This structure is perfect for placing focus on identifying exactly which device it is.

How does the single word "etsimämme" translate to the entire phrase "we are looking for"?

This uses the agent participle. It combines the verb etsiä (to look for) with the -mä participle ending and the first-person plural possessive suffix -mme (our). At the B2 level, this compact structure is heavily used instead of a full relative clause like laite, jota etsimme. It translates literally as "our looked-for device."

Why doesn't the word "rikki" have a normal adjective ending?

Rikki is an uninflected word indicating a broken state, rather than a standard adjective. It never changes its form to match the subject's case or number. If you wanted to use a standard inflecting adjective for "broken" instead of this state adverb, you would use the word rikkinäinen.

Is the comma before "joka" strictly necessary in Finnish?

Yes, absolutely! While English usually omits the comma before "that" in identifying relative clauses (like "the one that is broken"), Finnish grammar strictly requires a comma to separate the main clause from any relative clause starting with joka.