Learn German: Der Lärm belastet einen, aber die Ruhe beruhigt einen. - The noise burdens you, but the quiet calms you.

The pronoun man (meaning "one" or "people in general") can only be used as the subject of a sentence, which is the Nominative case. Here, Der Lärm (the noise) is the subject doing the action. The person receiving the burden is the direct object, so we must use the Accusative form of man, which is einen.

How do we know to use einen (Accusative) rather than einem (Dative) here?

The case is determined by the verbs. Both belasten (to burden) and beruhigen (to calm) require a direct object in the Accusative case. If the sentence used a verb that requires the Dative case, like helfen (to help), you would use einem instead.

The English translation says 'you', so why doesn't the German sentence use dich?

You could certainly say Der Lärm belastet dich, but that would mean the noise is burdening you specifically. By using einen, the German sentence is making a general statement that applies to anyone. In everyday English, we usually use a general "you" for this instead of the very formal "one".

Why doesn't the verb beruhigt move to the end of the clause after aber?