Learn Finnish: Kun törmään naapuriin, olemme molemmat olevinamme kiireisiä. - When I bump into the neighbor, we both pretend to be busy.

This is the quasi-construction, which expresses pretending. It pairs the conjugated verb olla (here, olemme) with the present active participle of the action you are pretending to do. That participle gets the suffix -vina- (or -vinä-) and a possessive suffix. Because the subject is 'we', we use the first-person plural suffix -mme, resulting in olevinamme.

Wait, why is the verb olla used twice here?

The first olemme is the auxiliary verb that the quasi-construction always requires. The second part, olevinamme, comes from the actual action being pretended, which in this case is also olla (to be), because you are pretending to be busy. If you were pretending to sleep, it would be olemme nukkuvinamme.

Why is the adjective kiireisiä in the partitive plural?

The adjective describes the plural subject 'we both' (olemme molemmat). In Finnish, when you use the verb olla to describe a plural subject with an adjective, that adjective complement must be in the partitive plural. The basic word kiireinen (busy) therefore becomes kiireisiä.

Why does naapuri take the illative case (naapuriin)?