Learn Turkish: Bitmedi partide onun öfkesi ve sitemi. - His anger and reproaches didn't end at the party.

This is a classic example of an inverted sentence (devrik cümle). In spoken Turkish or literary contexts, speakers often move the verb away from the end to emphasize it, or to add the rest of the sentence as an afterthought. Here, starting with bitmedi puts the focus entirely on the fact that the action didn't stop.

If bitmedi is the main verb, why is the subject onun öfkesi ve sitemi placed at the very end?

In a standard Turkish sentence, the subject comes first. By pushing it to the end after the verb, the speaker is treating it almost like a clarification. It is as if they are saying, It didn't end at the party... his anger and reproaches, that is. This creates a more dramatic, flowing, or colloquial rhythm.

Why do both öfkesi and sitemi have the -(s)i suffix? Could we just say onun öfke ve sitemi?

While you might occasionally hear native speakers drop the first suffix in casual speech, grammatically, both words belong to the pronoun onun (his/her). Therefore, both nouns in the joined phrase need their own 3rd-person possessive suffix: öfke-si (his anger) and sitem-i (his reproach).

What is the difference in nuance between öfke and sitem?

Öfke is straightforward: it means anger or rage. Sitem, however, translates to a reproach or gentle complaint. It expresses disappointment or a sense of being let down by someone close to you, usually without overt aggression. Combining them shows he was both outright angry and expressing personal disappointment.