Learn Finnish: Vuokralainen allekirjoitti uuden vuokrasopimuksen viikko sitten. - The tenant signed the new rental agreement a week ago.

This lesson focuses on the comitative case. How would we change the phrase uuden vuokrasopimuksen to the comitative if the sentence was "The tenant left with their new rental agreement"?

It would become uusine vuokrasopimuksineen. The adjective uusi takes the plural stem plus the -ne suffix (uusine). The noun takes the plural stem plus -ne- and the third-person possessive suffix -en (vuokrasopimuksineen). Even though it's one agreement, the comitative is always built on the plural stem.

Why do uuden and vuokrasopimuksen end in -n here?

They form the total object of the sentence. Because signing an agreement is a bounded, fully completed action, the object takes the genitive-accusative case (ending in -n) rather than the partitive case.

In the phrase viikko sitten (a week ago), why isn't viikko in the partitive or genitive case?

When using sitten to mean "ago," singular time words simply stay in the basic nominative case (like vuosi sitten or kuukausi sitten). You only use the partitive if you add a number greater than one, such as kaksi viikkoa sitten (two weeks ago), because the number kaksi forces the noun into the partitive.

How is the verb allekirjoitti constructed?