Learn Farsi: در حالی که دستگاه کار می‌کرد، نجار لبه‌ها را سنباده می‌زد. - While the machine was working, the carpenter was sanding the edges.

The phrase در حالی که (dar hâli ke) is used here to show two actions happening at the exact same time. When describing the past, we use the past continuous tense (which uses the می prefix) for both verbs to emphasize that the machine "was working" and the carpenter "was sanding" continuously and simultaneously.

What is the infinitive of سنباده می‌زد and how does this compound verb work?

The infinitive is سنباده زدن (sombâde zadan), which means "to sand". The word سنباده means "sandpaper", and زدن (to hit/strike) is a very common helper verb in Farsi used for applying something or using a tool. Since the action is in the past continuous ("was sanding"), the می prefix goes on the helper verb, making it سنباده می‌زد.

Why do we need the word را after لبه‌ها?

The word لبه‌ها (the edges) is the specific, definite thing being sanded. In Farsi, whenever you have a definite direct object (the exact receiver of the action), you must follow it immediately with the marker را. Because the sentence is about sanding "the edges" and not just "some edges" in general, را is required here.

Can در حالی که be used for anything other than two actions happening at the same time?

Yes! Just like the English word "while", در حالی که can also mean "whereas" to show contrast (for example: "I like tea, while my brother likes coffee"). In this sentence, however, the continuous verbs make it clear that it is talking about two simultaneous actions in time.