Learn French: Il conserve ce franc en cuivre sous prétexte qu'il est brillant. - He keeps this copper franc on the pretext that it is shiny.

It takes the indicative, which is why we see est and not soit. Even though a pretext implies the reason might be a flimsy excuse, the grammatical structure reports the reason exactly as the person stated it. Therefore, sous prétexte que is followed by the indicative mood.

Could I use Il garde instead of Il conserve?

Yes, garder and conserver are often interchangeable when they mean 'to keep'. However, conserver carries a slightly stronger nuance of preserving something carefully over time (like a collectible or an antique coin), which fits perfectly here.

Why is it en cuivre instead of de cuivre?

To say what a physical object is made of, you can actually use either en or de. En cuivre (made of copper) is the most common and standard way to express this in everyday French, while de cuivre sounds a bit more literary or formal.

Why is it qu'il est brillant and not que c'est brillant?

We use il est + [adjective] here because il refers back to a specific, previously mentioned noun: ce franc (which is masculine). If you used c'est brillant, it would mean something more general, like 'the situation is brilliant' or 'that is shiny' in an abstract sense.