Questions & Answers about Minun täytyy lajitella nämä erikseen.
Why does the sentence start with minun instead of minä?
The verb täytyy (must) is a necessity verb. In Finnish, the person who has to do something is always put in the genitive case. So, minä becomes minun.
Why is the object nämä instead of the partitive näitä?
With necessity structures like minun täytyy, a complete action takes its object in the basic form (nominative) instead of the usual object cases. Since you are sorting all of these specific items completely, you use the basic plural form nämä.
Since this lesson is about local cases like näissä (in these), why don't we use one here?
The word nämä (these) is the direct object of the verb lajitella (to sort). You are sorting the items themselves. You would use a local case like näissä only if you were doing something inside or attached to them, for example if you said there is a mistake in these.
Is the verb lajitella conjugated to match minun?
No, lajitella is in its basic dictionary form (the infinitive). When you use a necessity verb like täytyy, the action verb that follows it is never conjugated.
Where does the word erikseen come from?