Learn Russian: Тебе не нужно волноваться об этом.

After negative expressions of need like не нужно or не надо, Russian requires the imperfective infinitive to simply mean that an action is unnecessary. If you were to use a perfective verb here, it would completely change the meaning, implying that it is impossible to successfully complete the action.

Why is the sentence starting with тебе instead of ты?

Words expressing need or possibility, like нужно, надо, and можно, are impersonal in Russian. They do not take a standard subject. Instead, the person who experiences the need is put in the dative case. So, the nominative ты (you) becomes the dative тебе (to you).

Why is the preposition об used instead of just о?

The preposition о (about) changes to об when the word directly following it starts with a vowel sound. This makes the phrase easier to pronounce. Because этом starts with the vowel э, we must use об.

What case is этом in?

It is in the prepositional case. When the preposition о or об is used to mean 'about a topic' or 'concerning', the noun or pronoun that follows it must always take the prepositional case.