Learn Swedish: Polisen jagade tjuven genom parken. - The police chased the thief through the park.

It can be both! In Swedish, polisen often functions as a collective noun for the entire police force, taking singular verbs and adjectives. However, it is also the definite singular form of en polis (a police officer). Here, context implies the police force or officers in general, but if you need to be specific about multiple individual officers chasing the thief, you would use the plural poliserna.

I sometimes see igenom for the English word 'through.' Why is it genom instead of igenom in this sentence?

Genom is a preposition, meaning it must be followed by an object, which is parken in this sentence. Igenom is an adverb or verbal particle used when there is no object directly following it in the phrase, such as saying tjuven sprang igenom (the thief ran through).

Could I use the verb förföljde instead of jagade here?

You could, but it changes the nuance. Jagade means chased or hunted, implying a fast, active, and immediate attempt to catch the thief. Förföljde means pursued, followed, or even stalked, which can be slower, stealthier, or happen over a longer period of time without the immediate intent to pounce.

Why do we use genom parken (through the park) rather than över parken (across the park)?