Learn Spanish (Latin America): Ese hielo está muy opaco por sucio. - That ice is very opaque from being dirty.

In Spanish, you can use por followed directly by an adjective to mean "because of being [adjective]." Here, por sucio is a natural, concise way to express a cause. You could also say por estar sucio or porque está sucio, but dropping the verb is very common and sounds highly natural.

Why do we use está with opaco here instead of es?

Using estar highlights the current physical state of this specific ice, particularly because it is a temporary result of a change (it became opaque because it got dirty). If you used es opaco, it would describe a permanent, inherent characteristic of that material.

How would I ask "How opaque is that ice?" using the qué tan structure from this lesson?

You place qué tan directly before the adjective. The question would be ¿Qué tan opaco está ese hielo? In Latin America, this is the most standard way to ask about the degree or extent of a physical description.

I sometimes mix up ese and eso. Why is it ese hielo here?