Learn Finnish: Hän oli mitä korvaamattomin kollega työyhteisössämme. - He was an absolutely irreplaceable colleague in our work community.

This is the absolute superlative construction. While a normal superlative compares things, adding mitä before it translates to "absolutely [adjective]" or "the most [adjective] imaginable." It emphasizes an extreme degree without directly comparing the person to anyone else.

Since mitä usually takes the partitive case, why is kollega in the nominative here?

In this specific fixed structure (mitä + superlative adjective + noun), the noun always takes the nominative case. The word mitä here acts more like an emphasizing adverb than a pronoun, so it doesn't force the following noun into the partitive.

How do you break down a long word like korvaamattomin?

It stems from the verb korvata (to replace). Adding the suffix -maton (meaning "un-" or "without") makes korvaamaton (irreplaceable). To make it superlative, the -ton ending changes its stem to -ttom- before attaching the superlative marker -in, giving us korvaamattomin.

Why does työyhteisössämme end with -mme without the word meidän (our) in front of it?