Learn Farsi: دستیاری که مهارتش عالی است قطعه را جلا می‌دهد، در حالی که من سنباده می‌زنم. - The assistant whose skill is excellent polishes the piece, while I sand.

در حالی که (while/whereas) connects two simultaneous actions and is followed by verbs in the indicative mood. Because these actions are happening right now, both verbs take the present continuous form using the می prefix: جلا می‌دهد (polishes) and سنباده می‌زنم (I sand).

How does دستیاری که مهارتش mean 'The assistant whose skill'?

Farsi does not have a single word for 'whose'. Instead, you use a noun with the -i suffix (دستیاری), the connector که (that), and add a possessive ending to the following noun: مهارتش (skill + his/her). Literally, this is 'The assistant that his/her skill...'. The -i suffix here links the noun to a restrictive relative clause, which is why it translates as 'The assistant' rather than 'An assistant'.

What are the literal meanings of the two compound verbs جلا می‌دهد and سنباده می‌زنم?

Both are formed by combining a noun with a simple verb. جلا دادن pairs جلا (shine/polish) with دادن (to give), literally meaning 'to give shine'. سنباده زدن pairs سنباده (sandpaper) with زدن (to hit), literally meaning 'to hit sandpaper'.

Why is there a را after قطعه but not after سنباده?

(the piece) is a specific direct object receiving the action of being polished, so it requires the specific object marker . In contrast, is an inseparable part of the compound verb (to sand). It functions as part of the action itself, not as a separate object receiving an is acting upon.