Learn Farsi: شما نباید چیزی را در کشو پنهان کنید. - You must not hide anything in the drawer.

The word چیزی can mean either "something" or "anything". In a negative sentence like this one (because of نباید, meaning "must not"), it naturally translates to "anything". Farsi uses the same word for both depending on whether the entire sentence is positive or negative.

Why is the definite object marker را (râ) used after چیزی if "anything" is an indefinite pronoun?

While را usually marks definite objects, Farsi also uses it with indefinite pronouns like چیزی (anything) and کسی (anyone) when they are the direct object of the verb. Using چیزی را in a negative sentence is the standard way to emphasize "anything at all" as the object being acted upon.

Why does the main verb پنهان کنید (penhân konid) end in -ید (-id)?

The ending -ید matches the subject شما (you, formal/plural). The modal verb نباید (must not) forces the main verb that follows it to take the subjunctive form, which must be conjugated to match the person doing the action.

Why doesn't the verb have the subjunctive prefix بـ (be-), like پنهان بکنید?

This is a compound verb made of پنهان (hidden) and کردن (to do). In compound verbs formed with کردن, the subjunctive prefix بـ is very frequently dropped. While پنهان بکنید isn't wrong, it is much more natural and common to just say پنهان کنید.