A helpful tip: 'egg' is 'ei' in german, so that might help you remember the first letter at least :)
Subjectively Useful Trivia: Egg is the Norse form of the native English word Ey (pron.-"ay") or the plural Eyren (pron- ay-urn) [Like Iron but with an Ay sound], we borrowed the word Egg with the non-softened "g" from the Danish settlers in the early last millennia.
why is it an egg or one egg and not just simply egg as in, We are eating cheese and egg
So how i can know, that i have to use "et egg" or just "egg"? It does matter i will write "Vi spiser ost og egg" or "Vi spiser en ost og et egg"?
English and Norwegian work the same way in this case. Either you specify the amount, or you don't. You can translate directly from English to Norwegian. However, a cheese is normally quite big, so you are more likely to eat cheese than a cheese in one sitting, while eating an egg is a more normal amount.
The answer was "we are eating cheese..." How would you say in Norwegian, "we eat cheese...?" Is it different?
I don't think it is. Quite a few languages make no distinction between "We eat" and "We are eating".