Learn Farsi: ای دلاور، تو باید پادشاه را نجات بدهی.

ey is a formal vocative particle used to call out to someone, similar to "O" in English ("O brave warrior"). You won't hear it much in casual, everyday Farsi; people usually just state the person's name or title. It is mostly used in literature, poetry, speeches, or dramatic contexts like this one.

Since this lesson covers the vocative suffix -â, could I say delâvarâ instead of ey delâvar?

Technically yes, but the -â suffix is highly restricted in practice. It is mostly frozen in specific literary words like khodâyâ (O God) or dustâ (O friend). ey delâvar already sets a perfectly dramatic, epic tone; saying delâvarâ would sound overly archaic even for a storybook.

Why is the singular, informal to used to address the warrior instead of the polite shomâ, given the formal tone?

In modern social settings, shomâ is used for respect. However, in classical literature, religious texts, or epic fantasy (where you would address a "brave warrior"), the singular to is the standard way to address someone. It conveys a majestic or profound directness rather than everyday, informal familiarity.

Why does the verb nejât dâdan (to save) appear as nejât bedahi here?

The modal word bâyad (must) triggers the subjunctive mood for the verb that follows it. For compound verbs, only the verbal part takes the subjunctive prefix . So, (to give) becomes to match the "you" () subject.