Final answer:
The sentence provided pertains to the subject of English, where aspects such as grammar, narrative structure, and character actions are analyzed.
Step-by-step explanation:
The sentence you provided, "W'Kabi arrived early and ruined the surprise birthday party," is from the realm of English language and literature. It does not pertain to any mathematical calculations, historical events, scientific theories, or any other specific academic subject matter.
Instead, this line seems to come from a narrative or story context, which is usually discussed within English classes. When analyzing this sentence, one would possibly look into aspects such as grammar, narrative structure, and character development, making English the correct subject to categorize this question.