Learn Russian: В случае нарушения правил мы заблокируем дверь. - In case of a violation of the rules, we will block the door.

The compound preposition в случае (in case of) requires the noun that directly follows it to be in the Genitive case. Here, the neuter noun нарушение (violation) changes to нарушения.

Why is правил also in the Genitive case?

This is a common Russian structure called a Genitive chain. While нарушения is Genitive because of в случае, the word правил is Genitive plural simply because it belongs to the previous noun. It literally translates to a violation 'of the rules'.

How could we rewrite this sentence using the в случае, если structure from this lesson?

You would need to change the noun phrase into a full clause with a subject and a verb. For example, you could say: В случае, если вы нарушите правила, мы заблокируем дверь (In case you violate the rules, we will block the door). Notice how the noun нарушения turns into the verb нарушите, and правила shifts to the Accusative case as its direct object.

Wait, is дверь in the Nominative case here?

No, it is in the Accusative case because it is the direct object of the verb заблокируем. Feminine nouns ending in a soft sign, like дверь, belong to the 3rd declension and have identical Nominative and Accusative forms, which is why it looks unchanged.