Questions & Answers about Bilaget er i mappen.
Because bilaget is the definite singular form of et bilag.
In Norwegian, the is often added as an ending instead of being a separate word:
- et bilag = an attachment / document / appendix
- bilaget = the attachment / document / appendix
So bilaget means that a specific item is being referred to.
For the same reason: mappen is the definite singular form.
- en mappe = a folder
- mappen = the folder
So the sentence uses two definite nouns: bilaget and mappen.