Learn Finnish: Pienet kupit laitetaan sisäkkäin ennen tilaisuutta. - The small cups are put inside each other before the event.

The word is built from the root sisä- (inner or inside) and the mutual arrangement suffix -kkäin. This suffix shows how things are positioned relative to each other, so sisäkkäin literally means the cups are placed inside one another, or nested.

Since the cups are having an action done to them, why is Pienet kupit in the basic nominative plural form?

The verb laitetaan is in the passive voice, which focuses on the action being done rather than a specific person doing it. In Finnish, a whole plural object of a passive verb takes the nominative case. This means the object looks exactly like the standard plural form rather than taking a partitive or genitive ending.

Why does the word tilaisuus change to tilaisuutta in this sentence?

The word ennen acts as a preposition here, meaning before. It strictly requires the noun that follows it to be in the partitive case, which turns the basic word tilaisuus into tilaisuutta.

What is the background of the word tilaisuus?

In this context it translates to an event or occasion, but it is also the standard Finnish word for an opportunity. It is derived from the word tila, which refers to a space, room, or state.