Questions & Answers about او سهواً به دوست خود خیانت کرد.
What is the grammatical role of the two lines at the end of سهواً?
Those lines are an Arabic marker called tanvin, pronounced as an. In Farsi, it is added to Arabic nouns to turn them into adverbs. Here, the noun سهو (oversight) becomes the adverb سهواً (sahvan, accidentally). You will see this often in formal Farsi, though in casual speech someone might just say اشتباهی.
Why do we use the preposition به before دوست خود?
The compound verb خیانت کردن (to betray) requires the preposition به to introduce the person or thing being betrayed. You can think of the literal translation as doing betrayal to someone.
Could we attach a possessive ending and say دوستش instead of دوست خود?
Yes, دوستش is very natural. Using the reflexive pronoun خود emphasizes that he betrayed his own friend. It also removes ambiguity; if another person had been mentioned recently, دوستش could mean that other person's friend, but خود always refers back to the subject of the sentence.
This lesson focuses on the Past Perfect Subjunctive. How would we change this sentence to say it is possible that he had accidentally betrayed his friend?