Learn Hardcore Dutch: In het stadhuis helpt een vriendelijke ambtenaar ons met de papieren. - In the city hall a friendly civil servant helps us with the papers.

Why does helpt (the finite verb) come immediately after the prepositional phrase “in het stadhuis” and before the subject “een vriendelijke ambtenaar”?

This is due to the Dutch V2 (verb-second) rule in main clauses. In Dutch, the finite verb must occupy the second position in the sentence. If you start with an adverbial or prepositional phrase (“in het stadhuis”), the verb follows in slot two, and the subject comes third:

In het stadhuis (1st position)
helpt (2nd position, finite verb)
een vriendelijke ambtenaar (subject) …

Why do we use in het stadhuis instead of “op het stadhuis” or “naar het stadhuis”?

in indicates being inside a building or enclosed space.
op would mean “on top of” a surface.
naar expresses movement toward a place (“to the town hall”).
Since the sentence describes someone being inside the town hall and helping you there, in het is correct.

Why is stadhuis preceded by het, even though stad is a de-word?

Stadhuis is a compound noun (stad + huis). In Dutch, the head of the compound determines the gender. Here huis is neuter (het huis), so the whole compound takes het:
stad (de-word) + huis (het-word) → het stadhuis.

Why does the adjective vriendelijke have an -e ending when it comes before ambtenaar?

Dutch adjectives preceding nouns normally take an -e ending except in one case (indefinite neuter singular). Rules:

For de-words (common gender, singular or plural) + any article (de, een, geen,…): adjective gets -e.
For het-words (neuter) with de, adjectives get -e.
For het-words (neuter) with een (indefinite), adjectives do not get -e.
Here ambtenaar is a common-gender noun (de-word), and we use een → vriendelijke ambtenaar.

Why is the article een used instead of de? Could we say de vriendelijke ambtenaar?

een = indefinite article (“a/an”), introducing any friendly official.
de = definite article (“the”), referring to a specific official already known to speaker and listener.
You can use de vriendelijke ambtenaar if you have already mentioned or identified the official.

Why is ons used here and not wij?

wij is the subject pronoun (“we”).
ons is the object pronoun (“us”).
In our sentence the people (“us”) are the object of the helping action, so we need ons.

Why do we say helpt ons met de papieren instead of just helpt ons de papieren?

The verb helpen when meaning “to assist” often requires the preposition met before the thing you’re being helped with:
iemand helpen met iets = to help someone with something
Without met, the sentence would sound incomplete or change meaning (in Dutch you don’t leave out met in this context).

Why is the object pronoun ons placed before the prepositional phrase met de papieren?

Pronoun objects in Dutch generally appear immediately after the finite verb (or after “te + infinitive” in subordinate clauses), before any prepositional or adverbial phrases:
… helpt ons (object pronoun) met de papieren (prepositional object).

Why is papieren in the plural here? Couldn’t it be singular papier?

papier (het papier) as a material is uncountable, used without plural to mean “paper.”
papieren (de papieren) as documents is a countable plural.
Here you mean “the papers/documents” (e.g. forms, permits), not the material. Hence de papieren with the plural -en ending.

What nuance does ambtenaar carry, and could you use beambte or klerk instead?

ambtenaar = a civil servant, official working for the government or municipality.
beambte is more old-fashioned or formal, also meaning “official/clerical employee.”
klerk typically means “clerk,” someone who does office work, but not necessarily a government official.
In a town-hall context ambtenaar is the most natural choice.