Learn German: Drittens dürfen Sie nichts wegen eines Fehlers aufschieben. - Thirdly, you must not put anything off because of a mistake.

It is formed by attaching the suffix -ns to the ordinal number dritte (third), which applies the -ens enumeration pattern from this lesson. This turns it into an adverb meaning "thirdly." Because drittens occupies the first position in the sentence, the conjugated verb dürfen must immediately follow it in the second position.

The English translation says "must not," but the German sentence uses dürfen. Why not use müssen?

This is a classic trap! In English, "must not" expresses a strict prohibition. In German, prohibition is expressed with nicht dürfen (or here, nichts dürfen - to not be allowed to do anything). If you used nichts müssen, it would mean "you do not have to do anything" (a lack of obligation, not a rule).

Why does ein Fehler change to eines Fehlers in this sentence?

The preposition wegen (because of) requires the genitive case in standard German. Since der Fehler is a masculine noun, the indefinite article becomes eines and the noun gets an extra -s at the end. While you might hear the dative case (wegen einem Fehler) in casual speech, the genitive is the correct written form.

aufschieben (to put off) is a separable verb. Why is it in one piece at the end of the sentence?