Learn Swedish: I smedjan smider vi metallen platt. - In the forge, we forge the metal flat.

Even though the object metallen is in its definite form (the metal), an object predicative never takes a definite ending. It works just like a normal predicative after the verb to be. You would say metallen är platt, so you also say vi smider metallen platt.

Does platt end in -t here because it is an ett-word form?

No, the base form of this adjective for en-words is actually platt. Because the root word already ends in a double consonant plus t, the ett-word form does not add another t; it stays exactly the same (ett platt tak).

Why is it smider vi instead of vi smider?

This is a quick reminder of the V2 (verb-second) rule. Because the sentence begins with the location phrase i smedjan, the verb smider must take the second position in the sentence, pushing the subject vi right behind it.

I smedjan and smider look very similar. What is the connection?