Learn Finnish: Molemmat osallistujat saivat kutsun. - Both participants received an invitation.

In Finnish, when multiple people each receive exactly one of something, the object is usually singular. If you used the plural kutsut, it would imply that they each received several invitations, or that they collectively received a specific batch of invitations.

Why is the object kutsun in the genitive-accusative case instead of the partitive?

The action of receiving (saada) is a completed, bounded action, and an invitation is a countable whole object. Because this is a standard affirmative sentence with a subject, this total object takes the genitive-accusative ending -n.

How does the word molemmat behave if we add this lesson's target phrase, lukuun ottamatta (except), to say "except both participants"?

The postposition lukuun ottamatta usually governs the partitive case. Even though molemmat means "both" (implying two), it declines using plural case endings. Therefore, you would use the partitive plural to say molempia osallistujia lukuun ottamatta.

What is the root structure behind the noun osallistujat?

It is built from the verb osallistua (to participate). By taking the verb stem and adding the agent suffix -ja (which is like the English "-er"), you get osallistuja (participant). The -t at the end simply makes it plural.