Wouldn't smørbrød sort of be butter bread, since smør is butter?
- Cut a slice of bread (brød)
- Spread butter on it (smør)
- Add toppings (...)
- Eat your sandwich (smørbrød)
After you're done with the second step, and before it is made into a proper sandwich, it could be described as "butter + bread". Smørbrød is a compound word made from smør and brød but when these two are used in et smørbrød, it's translated as "a sandwich" instead of "a butterbread".
"Et" marks a neuter gendered noun, like "et brev" or "et dyr" (eng. "a letter" and "an animal"). "En" is a masculine noun marker (i.e. "en gutt," eng. "a boy") but can also be used for feminine nouns. "Ei" is also a feminine noun marker, but you can use "en" for both masculine and feminine nouns in most cases.
Correct me if i'd be wrong: At the frase "Du spiser et smørbrød" is pronounced like that -> "Dus pisser ets morbrea" 'cause the first letter of the next one joins with the previous one when speaking. It's like a rule or because when the next word beggin with s letter, you match them.