Learn Finnish: Älkäämme olko armollisia vastarinnalle, sillä olemme häikäilemättömiä! - Let's not be merciful to the resistance, for we are ruthless!

The negative 1st person plural imperative uses the auxiliary verb älkäämme (let us not). The main verb then takes a special connegative ending: the consonant stem of the verb plus -ko or -kö. For olla, the stem is ol-, giving us olko.

Is älkäämme olko used in everyday spoken Finnish?

Rarely. The älkäämme + -ko/-kö structure is strictly formal or literary. In everyday spoken Finnish, you express "let's not" by using the negative passive: ei olla. So, casually, this would simply be ei olla armollisia.

Why are the adjectives armollisia and häikäilemättömiä in the partitive plural?

They act as subject complements describing "we" (implied by älkäämme and olemme). When the subject of the verb olla (to be) is plural, the adjectives describing that subject are almost always put into the partitive plural.

How does the word häikäilemättömiä get its shape from the basic adjective?