Learn Finnish: Karhun poikanen leikkii metsässä. - The bear's cub plays in the forest.

The suffix -nen is a diminutive, typically used to mean "small" (like kukkanen for a small flower). When added to poika (boy/son), it creates the lexicalized word poikanen, which specifically means an animal's cub or offspring.

If I want to talk about multiple cubs, how does the word poikanen decline?

Nouns ending in the suffix -nen change their stem to -se- before most endings are added. To form the nominative plural, you add -t, so poikanen becomes poikaset (cubs).

I often see animal cubs written as compound words. Is karhunpoikanen also correct instead of Karhun poikanen?

Yes, both are perfectly fine. Writing it as one compound word (karhunpoikanen) treats it as a single concept (a bear-cub), whereas keeping them separate (Karhun poikanen) uses a genitive structure that can emphasize the specific relationship (the bear's cub). You will encounter both forms in Finnish.

Does the verb leikkiä require any special location cases here?