Er öffnete eine Flasche Rotwein
Why is the indefinite article eine when Rotwein is a masculine object?? Why is it eine and not ein?
The article is referring to the direct object of the sentence. In this case that's the bottle, or "die Flasche". The "Rotwein" is just describing the bottle. He is not doing anything to the red wine itself, he's opening the bottle, not the wine.
Hope that helps.
Then it should have been written in minor case, not with a capital letter ;)