Learn Finnish: Poliisit vahtivat rakennusta vuoroittain. - The police officers guard the building in shifts.

It uses the distributive suffix -ittain, which is the focus of this lesson. You take the plural stem of vuoro (shift), which is vuoroi-, and attach -ttain. This suffix expresses how an action is divided or distributed, translating here to "in shifts" or "shift by shift."

Why does this word use the -ittain ending rather than -ittäin?

This is a classic example of vowel harmony. Because the root word vuoro contains the back vowels u and o, the suffix must also use the back vowel variant -ittain. If the root word only had front vowels (like y, ä, ö) or neutral vowels, it would take -ittäin.

Why is the object rakennus in the partitive form rakennusta?

The verb vahtia (to guard or watch) is a partitive verb. It describes an ongoing, continuous action rather than an action that produces a finalized result. Therefore, its object must be in the partitive case.

Why does the sentence use the plural poliisit instead of just the singular poliisi?

In Finnish, the singular poliisi is often used as a mass noun to refer to the police force as a whole institution. However, the plural poliisit is used here to specifically point out multiple individual police officers, which makes sense since they are taking turns guarding.