Learn German: Erstens müssen Sie jeden Vorsatz sofort aufschreiben. - Firstly, you must write down every resolution immediately.

Erstens is an adverb meaning "firstly" or "in the first place," created by adding the suffix -ns to the ordinal number erste (first). You use it specifically for enumerating points in an argument or a list. By contrast, erste is an adjective that modifies a noun (like der erste Vorsatz), and zuerst refers to chronological order meaning "initially" or "first of all."

Why does the verb müssen come right after erstens, instead of after Sie?

Because erstens is an adverb occupying the first position in the sentence. German strictly follows the V2 (verb-second) rule in main clauses, so the conjugated verb (müssen) must take the second slot, pushing the subject (Sie) to the third position. English allows a comma after "Firstly" followed by normal subject-verb order, but German requires the verb immediately after the adverb.

Why do we use jeden instead of jede or jeder before Vorsatz?

The noun Vorsatz (resolution or intention) is masculine (der Vorsatz). Because it acts as the direct object being written down in this sentence, it must be in the accusative case. The word jeder (every) takes the standard masculine accusative ending -en, becoming jeden.

Aufschreiben is a separable verb. Why is it kept together at the end of the sentence?