Learn Swedish: Polisen väntade tyst med knytnävarna på bordet. - The police officer waited silently with his clenched fists on the table.

In Swedish, when a body part or piece of clothing is used in a descriptive phrase like this one (med + noun + location), you almost always use the definite form of the noun rather than a possessive pronoun. It is automatically understood from the context that the fists belong to the subject.

How do we know polisen means "the police officer" here rather than "the police" in general?

The word polis can refer collectively to the police force, but it is also the standard noun for an individual police officer (en polis). Because having clenched fists on a table is a physical action performed by an individual, context tells us that polisen here means the specific police officer.

Is tyst acting as an adjective or an adverb in this sentence?

It is acting as an adverb, modifying the verb väntade to describe how the officer waited (silently). In Swedish, you usually form an adverb by taking the neuter t-form of the adjective. Because tyst already ends in a t, the adjective and adverb forms look exactly the same.

How is the compound word knytnävarna built?