Learn Farsi: این کس دیگری بود که پیروزی را خراب کرد. - It was someone else who ruined the victory.

This sentence uses the structure این ... بود که to highlight who did the action. The word این acts like the English dummy pronoun 'it', followed by the focused phrase کس دیگری, the past tense verb بود, and finally که to introduce the rest of the clause.

Why is there an ی at the end of دیگر in the phrase کس دیگری?

That is the indefinite suffix, indicating 'a' or 'some'. In Farsi, when a noun is modified by an adjective, this indefinite suffix often attaches to the end of the adjective rather than the noun itself. Here, it turns 'another person' into 'someone else'.

Why do we need را after پیروزی?

The word را is the definite direct object marker. Because the sentence refers to a specific, known event (the victory), we must use را after پیروزی to show it receives the action of the verb خراب کرد.

Does که translate to 'who' or 'that' in this sentence?

It translates best to 'who' because it refers to a person, but Farsi is much simpler than English here. It uses که as a universal connector for both people and things, so you never have to choose between 'who', 'which', or 'that'.