Learn Finnish: Jopa opettaja on turhautunut hiomaan samaa tekstiä. - Even the teacher is frustrated with polishing the same text.

Adjectives expressing emotional fatigue or saturation, like turhautunut (frustrated), väsynyt (tired), and kyllästynyt (fed up), require the illative case for the thing causing the feeling. Because the trigger here is an action (to polish), we use the 3rd infinitive illative form, which ends in -maan or -mään.

How is the word turhautunut formed?

It originates from the adjective turha (useless or vain). This forms the verb turhautua (to become frustrated). Turhautunut is the past active participle (the NUT-participle) of that verb, acting here as an adjective meaning "frustrated".

Why is the object samaa tekstiä in the partitive case?

The verb hioa (to polish or refine) takes a partitive object here because the action is viewed as an ongoing, incomplete process. The teacher is in the middle of working on the text, rather than having completed a finished product.

I know hioa usually means "to sand" or "to grind". Is it normal to use it for writing?

Yes, just like the English verb "to polish", hioa is very commonly used figuratively to mean refining or perfecting something abstract, such as a text, a plan, or a specific skill.