Learn Hardcore Finnish: Menemme kirjakauppaan, koska siellä on ilmainen tapahtuma. - We are going to the bookstore because there is a free event there.

menemme is the first-person plural present tense of the verb mennä (to go). The ending -mme marks “we,” so menemme literally means “we go” or “we are going.”
By contrast, mennään is the passive form (3 sg) of mennä but used colloquially as a first-person-plural imperative/suggestion: “let’s go.”

menemme = neutral statement (“we go / we are going”)
mennään = informal suggestion (“let’s go”)

Why is kirjakauppaan in that particular case?

kirjakauppaan is in the illative case, which expresses movement into something. Here it marks the destination “to the bookstore.”
Formation rule for illative (singular) on words ending in -a: double the a and add -n:

kirjakauppa → kirjakauppaa + n → kirjakauppaan
If you were already inside the bookstore, you’d use the inessive case: kirjakaupassa (“in the bookstore”).

What does koska mean, and why is it used here?

koska is a subordinating conjunction meaning because. It introduces a reason clause explaining why “we go to the bookstore.”

It must be followed by a subject and verb (e.g. siellä on…).
In everyday speech you might also meet sillä (“for (that) reason”) or siksi että (“for that reason that”), but koska is the standard way to say “because” when giving a factual reason.

Why is there a comma before koska? Is it required?

In Finnish, putting a comma before koska is optional.

You can write Menemme kirjakauppaan koska… without a comma.
Writers often include a comma to improve readability or avoid ambiguity, but it’s not a strict rule.

What is siellä, and why is it used instead of repeating “kirjakauppa”?

siellä is a locative pronoun/adverb meaning there (at that place). It refers back to the bookstore without having to repeat kirjakauppa.

Grammatically, it’s an adverbial form of se in a locative sense.
Placed at the start of the subordinate clause, it elegantly avoids repetition.

Why is ilmainen tapahtuma in the nominative singular, not in the partitive?

In existential clauses with olla (“there is/are”), singular items typically appear in the nominative case:

siellä on ilmainen tapahtuma = “there is a free event.”
Use the partitive only for unspecific or plural instances, e.g. siellä on ilmaisia tapahtumia (“there are free events”).

What’s the difference between mennä kirjakauppaan and käydä kirjakaupassa?

mennä kirjakauppaan (illative) = to go to the bookstore (focus on the act of going, often planning to stay or browse)
käydä kirjakaupassa (inessive) = to visit / stop by the bookstore (often implies a short visit or a habitual action)
Use käydä when you want to stress “visiting” (as a stop or routine), and mennä when you talk about the journey or destination itself.