Learn Finnish: Me emme osallistuneet tapahtumaan. - We did not participate in the event.

In negative past tense sentences, the main verb takes a past participle form. Because the subject is plural (me, meaning we), we must use the plural past participle ending -neet. If the subject was singular (like minä), it would be osallistunut.

How do we form the -neet ending for the verb osallistua?

Osallistua (to participate) is a Type 1 verb. To form its past participle, we take the infinitive stem by dropping the final -a (giving us osallistu-) and add -neet for the plural, creating osallistuneet.

Why is the word for event (tapahtuma) in the illative form tapahtumaan?

While in English we say we participate in something, the Finnish verb osallistua strictly requires the illative case (the into case). You can think of it as entering or joining into an activity. That is why we add the illative ending -an to get tapahtumaan instead of using the inessive case.