Learn Finnish: Hän on johtajana toimistossa. - She is acting as a manager in the office.

This is the essive case in action! Adding -na translates to "as a manager" or "in the role of a manager." If you say Hän on johtaja, it means being a manager is her permanent job or title. Using johtajana highlights that she is currently acting in that capacity or filling that specific temporary role.

How do we choose between the -na and -nä endings for the essive case here?

It comes down to vowel harmony. Because the base word johtaja contains the back vowels o and a, the ending must also use a back vowel. Therefore, we attach -na instead of -nä.

How is the word johtaja formed?

It is built from the verb johtaa, which means "to lead" or "to direct." The -ja suffix turns a verb into a noun describing the person who does that action, exactly like adding "-er" in English. A johtaja is simply a "leader" or "manager."

As a quick refresher, why use toimistossa rather than toimistolla?

The -ssa ending means "in" or "inside." We use toimistossa to emphasize that she is physically inside the enclosed office space. If you used toimistolla, it would mean she is "at the office" in a more general sense, perhaps just on the grounds or busy at work.