"She has a beer."
Translation:Hun har en øl.
'en øl' is a countable noun, meaning contained in something and can be counted.
'et øl' is an uncountable noun, meaning the type of beer.
Sort of. If you are drinking one, it's in glasses on the table, it's en øl. If you are talking about quantities, a brand or beer as a concept, it's et øl. This characteristic is just for beer as far as I know, and not other beverages.