Breakdown of Lagið er samið af trommuleikaranum.
vera
to be
lagið
the song
af
by (agent of a passive)
semja
to compose
Questions & Answers about Lagið er samið af trommuleikaranum.
Why do we use af here to express who did the action?
In Icelandic passive sentences, the agent (the person actually doing the action) is introduced with the preposition af. This preposition always triggers the dative case, which is why the noun following it is in its dative form.
Why is the participle spelled samið here?
In a passive sentence, the past participle acts like an adjective and must agree with the subject in gender and number. Since the subject lagið is a neuter singular noun, the past participle of the verb semja (to compose) takes the neuter singular form samið.
How do we break down the ending on trommuleikaranum?
The base word is the weak masculine noun trommuleikari (drummer). Because the preposition af requires the dative case, the noun becomes trommuleikara. Finally, we attach the dative masculine definite article -num to specify that it was done by the drummer, not just any drummer.
What is the infinitive of samið and what else is it commonly used for?
The infinitive is semja. While it means to compose or to write in the context of music or literature, it is also highly common in daily news and business contexts where it means to negotiate or to reach a formal agreement.