Learn German: Du musst die Hausordnung des Besitzers lesen. - You must read the owner's house rules.

This is the genitive case, which shows possession. Because Besitzer (owner) is a masculine noun, the article changes to des, and we add an -s to the end of the noun. It translates literally as 'of the owner'.

The translation says 'house rules' in the plural, but die Hausordnung is singular. Why?

In German, die Hausordnung is a singular compound noun made from das Haus (house) and die Ordnung (order or regulation). German uses this singular collective term to refer to the whole set of rules for a building, whereas English prefers the plural word 'rules'.

How do we know the gender of a compound noun like Hausordnung?

The gender of any German compound noun is always determined by its final word. Here, the last word is Ordnung. Because all German nouns ending in -ung are feminine, the entire compound noun takes the feminine article die.

Why is the verb lesen at the very end of the sentence?

This sentence uses the conjugated modal verb musst (must). In a regular German main clause, the modal verb takes the second position, and the main action verb (lesen) is pushed to the very end of the sentence in its infinitive dictionary form.