Learn Finnish: Yrityksen tulee säilyttää jokainen tosite tilikauden jälkeen. - The company must keep every voucher after the financial year.

Both express necessity, but tulee followed by the first infinitive (säilyttää) is a highly formal construction. You will see it frequently in official documents, contracts, and legal rules. Just like with täytyy, the subject (yrityksen) must be in the genitive case.

Why is the object jokainen tosite in the basic nominative form instead of the genitive jokaisen tositteen?

This is a key rule for all necessity sentences: when a sentence expresses necessity (using verbs like tulee, täytyy, or pitää), a total object is always in the nominative case rather than the genitive. Since tulee säilyttää is a necessity structure, the object takes the nominative form.

Could I use the verb pitää instead of säilyttää to mean 'to keep' here?

In this business context, säilyttää is much better. It specifically means to preserve, store, or keep something safe over a period of time, such as official records. The verb pitää is used for physically holding something or keeping a promise, but it does not fit the idea of document retention.

What is the difference between kuitti and tosite?