Learn Finnish: Virtuaalinen peli on tärkeä osa elektronista urheilua. - The virtual game is an important part of electronic sports.

The noun osa (part) requires the concept it belongs to to be in the partitive case. Whenever you want to say 'a part of [something]' in Finnish, you should remember the structural formula: osa + partitive.

Could I write virtuaalinen peli as a single compound word like virtuaalipeli?

Yes, both are grammatically correct. Using the full adjective virtuaalinen emphasizes the quality of the game being virtual, while the compound word virtuaalipeli treats it as a fixed category of game. Both structures are very common.

Is 'electronic sports' always written as two separate words in Finnish?

Not always. While the formal term used here is the adjective-noun pair elektroninen urheilu, it is very frequently abbreviated in everyday written and spoken usage to the compound word e-urheilu.

How would I apply this lesson's formal imperative to say 'Let's be an important part of electronic sports!'?

You would use the first person plural imperative of the verb olla (to be), which is olkaamme. The sentence becomes: Olkaamme tärkeä osa elektronista urheilua! Keep in mind that in everyday spoken Finnish, you will almost always hear the passive form used to express 'let us' instead: Ollaan tärkeä osa...