Learn Russian: Юрист выступает как посредник.

When you use как to mean "in the capacity of," the noun that follows it must match the case of the noun it refers back to. Because юрист is the subject of the sentence (in the nominative case), посредник must also be in the nominative case. If the lawyer had been the direct object, both would be in the accusative case.

How would this sentence change if we used в качестве instead of как?

The phrase в качестве (in the capacity of) always requires the noun that follows to be in the genitive case. If we used it here, the sentence would become: Юрист выступает в качестве посредника.

The verb выступает usually means "performs" or "gives a speech." Why is it translated as "acts" here?

While выступать often refers to public speaking or performing on stage, it is the standard verb used in formal contexts to mean "to act as," "to serve as," or "to step into the role of." You will frequently see it paired with как or в качестве to express taking on a specific role.

Why is there no comma before как in this sentence?

In Russian, you generally use a comma before как when it introduces a direct comparison (e.g., "brave, as a lion"). However, when как simply introduces a role or profession meaning "in the capacity of," you do not put a comma before it.