Questions & Answers about Zij twijfelt over de opponent en rekent niet op haar.
Why do we use op haar instead of the pronominal adverb erop?
Because de opponent refers to a person. In Dutch, you cannot use er + preposition (like erop) to refer to humans. For people, you must always use a preposition plus a personal pronoun, which gives us op haar (or op hem).
How would the sentence change if she were doubting an inanimate object, like a strategy?
If she were doubting a strategy (a thing, not a person), you would have to use a pronominal adverb. The second half of the sentence would change to en rekent er niet op.
Why is there a preposition after twijfelen when English just says 'doubts the opponent'?
In Dutch, twijfelen (to doubt) requires the fixed preposition over to connect to an object. You always twijfelen over something or someone.
Why is the subject zij not repeated after en?
Because the two clauses are joined by the coordinating conjunction en (and) and share the exact same subject. Just like in English, you can drop the subject in the second clause to avoid sounding repetitive, rather than saying en zij rekent niet op haar.