Learn Farsi: آنها به عنوان داوطلب در بیمارستان خدمت می‌کنند. - They serve as volunteers in the hospital.

In Farsi, when using be onvân-e to state a role or capacity, the noun following it typically stays singular, even if the subject is plural. You are essentially saying they serve in the role of volunteer, so adding a plural marker like -hâ to dâvtalab is unnecessary and would sound unnatural.

What does the word onvân literally mean in this structure?

The word onvân literally means 'title', 'heading', or 'designation'. Therefore, the phrase be onvân-e translates literally to 'by the title of'. This makes it clear why this specific preposition is used for official capacities, jobs, or formal roles.

Could I use mesl-e (like/as) instead of be onvân-e to mean 'as volunteers'?

No, using mesl-e changes the meaning entirely. Mesl-e indicates similarity ('like'). If you said mesl-e dâvtalab, it would imply they are acting like volunteers (perhaps doing free work) but are not officially recognized as such. Be onvân-e confirms that 'volunteer' is their actual, designated role.

Is there a hidden ezafe connecting be onvân to dâvtalab?

Yes! When spoken, it must be pronounced be onvân-e dâvtalab. The ezafe (the unstressed 'e' sound) is grammatically required here to link the phrase to the noun it modifies, acting much like the word 'of' in the literal translation 'by the title of'.