Learn Spanish (Spain): Los carbohidratos no son malos, sino que nos dan energía. - Carbohydrates are not bad, but rather they give us energy.

Both mean "but rather" after a negative statement. You use sino que because the second part of the sentence has a conjugated verb (dan). If there were no newly conjugated verb in the second part (for example, "Not bad, but good" -> No son malos, sino buenos), you would just use sino.

Could I use pero instead of sino que in this sentence?

No, because the first part of the sentence is negative (no son malos) and the second part directly corrects or replaces that idea. Pero just adds information or a limitation (like "They are good, but they have calories" -> Son buenos, pero tienen calorías). When you are expressing "Not A, but rather B", you must use sino or sino que.

Who is the subject of the verb dan in the second half?

The subject is still los carbohidratos. Even though there is a new clause introduced by que, the subject carries over. So, "they give us" is translated as nos dan, with the verb conjugated in the "they" form.

Why is the pronoun nos placed before the verb dan?

Nos is an indirect object pronoun meaning "to us". In Spanish, object pronouns are placed immediately before the conjugated verb. So "they give us energy" becomes nos dan energía.