Learn German: Sie verabscheut es, wenn der Zug Verspätung hat. - She despises it when the train is delayed.

The word es acts as a placeholder (Korrelat) for the upcoming wenn clause. The verb verabscheuen (to despise) strongly expects a direct object. Since the actual object of her hatred is the entire second half of the sentence, German inserts es in the main clause to fill that grammatical gap. This is highly common with verbs of emotion like lieben, hassen, and verabscheuen.

Would it be wrong to say dass der Zug Verspätung hat instead of wenn?

Both are grammatically correct but carry slightly different nuances. Using wenn implies a recurring situation or condition (whenever the train is delayed). If you used dass, it would refer to a specific, factual instance (the fact that the train is delayed right now).

Why does the German sentence say Verspätung hat (has delay) rather than translating the passive English structure 'is delayed'?

While you could technically use the adjective and say verspätet ist, German speakers overwhelmingly prefer the noun phrase Verspätung haben (to have a delay) when talking about trains, flights, or even people running late. It is the most natural, idiomatic way to express this idea.

I don't see a ge- prefix. How would you form the past participle of verabscheuen if this sentence were in the past tense?