Learn Turkish: Kütüphaneci yokken sisteme erişmeliyiz ve kitapları uzatmalıyız. - We must access the system and extend the books while the librarian is absent.

This sentence is in the active voice because we have a specific subject ("we"). If you wanted to express this as a passive (like "The system must be accessed"), you would use the passive verbs erişilmek and uzatılmak: Sisteme erişilmeli ve kitaplar uzatılmalı. Notice how sisteme keeps its dative case even when the sentence becomes passive!

Why does the sentence use sisteme (to the system) instead of the direct object sistemi?

The verb erişmek (to access / to reach) always requires the dative case (-e/-a) in Turkish, indicating the destination you are reaching toward. Think of it as "reaching to the system."

How does yokken work here? I'm used to seeing -ken attached to verbs.

The suffix -(y)ken (while / when) can also attach directly to adjectives, nouns, or existence words. Here, it attaches to the word yok (non-existent / absent). Kütüphaneci yokken translates smoothly to "while the librarian is absent" or "when there is no librarian."

Does kitapları uzatmak literally mean "to make the books longer"?

Uzatmak generally means "to extend," "to prolong," or "to stretch out." In a library context, just like in English where you "extend" a deadline or a loan period, kitapları uzatmak is the standard phrase for "renewing the books."