Indirect questions and mandatory "som"
An indirect question is embedded inside another sentence, like "I wonder who..." or "We do not know what...".
In Swedish, if an interrogative word like vem (who) or vad (what) is the subject of the embedded clause, you must add the word som directly after it.
For example, in the direct question Vem skapar gatukonsten? (Who creates the street art?), vem is the subject. When turned into an indirect question, som becomes mandatory:
| No one knows who creates the street art. | Ingen vet vem som skapar gatukonsten. |
| We are discussing what makes the graffiti controversial. | Vi diskuterar vad som gör graffitin kontroversiell. |
| We do not know who leaves the spray can. | Vi vet inte vem som lämnar sprayburken. |
Using "vilken" as a subject
This rule also applies to vilken (which) and its forms (vilket, vilka). When vilken and its attached noun form the subject of the embedded clause, som immediately follows the noun.
| It is a secret which pseudonym hides the signature. | Det är en hemlighet vilken pseudonym som döljer signaturen. |
| I can see which stencil does not have the signature. | Jag kan se vilken schablon som inte har signaturen. |
Note: *som is only added when the question word is the subject doing the action. If the question word is the object (e.g., "I don't know who you saw", where "you" is the subject), you do not use som.*