Learn Russian: Нельзя отвлекаться, делая это упражнение. - One must not get distracted while doing this exercise.

Делая is an imperfective verbal adverb meaning "while doing." To form it, take the "they" form of the present tense (делают), drop the -ют, and add -я. It shows an ongoing background action happening at the same time as the main action (отвлекаться).

The rule says verbal adverbs must share a subject with the main verb, but this sentence has no subject! Why is делая allowed here?

Great catch! It is true that the main verb and the verbal adverb usually need the same subject. However, there is an exception for impersonal sentences (like ones using нельзя, надо, or можно) followed by an infinitive. Since the implied doer of отвлекаться (anyone/you) is the same implied doer of делая, the sentence is perfectly grammatical.

Why is the imperfective verb отвлекаться used after нельзя instead of the perfective отвлечься?

When you use нельзя with an imperfective verb, it means "one must not" or "you shouldn't" (a rule or prohibition). If you used the perfective verb (нельзя отвлечься), it would mean "it is impossible to get distracted" (for example, because the task is just too gripping).

Is the comma before делая required?