This is the 2929th most frequent Swedish word.
en
"En" in Swedish means "a" or "an" (indefinite article) or "one" (numeral).
In this sentence, 'en' serves as an indefinite article, equivalent to 'a' or 'an' in English, modifying the noun 'bok' (book).
Jag köpte en bok igår.
I bought a book yesterday.
In this example, 'en' is an indefinite article modifying the noun 'vän' (friend), indicating he represents one among various potential friends.
Han är en god vän till mig.
He is a good friend of mine.
Here, 'en' is used as an indefinite article accompanying the noun 'fågel' (bird), marking it as nonspecific or unnamed.