Pero vs sino que

Question

Hi, 
 
I stumble over pero, sino que and sino and the following example is a good one to drive me crazy. 
Elia no va a empezar el curso este año ________ va a tener un año sabático.
Pero vs Sino Que. 
For the example above both „pero“ and as weln „sino que“  were flagged as correct  and wrong in Kwiziq! 
I have checked the rules more than once and verified that with OpenAI, Mistral AI, Claude, etc and all came across with rules and explanations and all of the AI explained to use pero and when I asked again they said use sino que. So it would be nice to get your answer on it and to correct and remove the wrong answers. 
Thanks,
Bernd

Silvia Piriz · Accepted Answer

Hola Bernd E.
Thank you for your patience, this is a great example to clarify.
In your sentence, the correct option is:
Ella no va a empezar el curso este año, sino que va a tener un año sabático.
We use sino (que) when the first clause is negative and the second clause corrects or replaces that idea. The structure is:
No X, sino (que) Y
Here, no va a empezar el curso is the negative statement and the second part replaces that idea with a different action: va a tener un año sabático. Because the second part contains a conjugated verb (va a tener), we must use sino que.
If the second part were just a noun (not a full clause), we would use sino without que, for example:
No es profesora, sino estudiante.
We do not use pero here because pero expresses contrast, not correction. Your sentence does not contrast two independent facts, it replaces one idea with another. That’s why sino que is the grammatically correct connector in this case.
Saludos
Silvia