Questions & Answers about Bygningen er svær.
Because Norwegian usually puts the definite article at the end of the noun.
- bygning = building
- bygningen = the building
So -en here is the definite ending for a singular common-gender noun.
Because in Norwegian, definiteness is often built into the noun itself instead of being a separate word before it.
English says:
- the building
Norwegian often says:
- bygningen
The ending -en already carries the meaning of the.