Learn Icelandic: Lesandinn greinir skáldaverkið er hún ritaði. - The reader analyzes the literary work which she wrote.

In formal written Icelandic, er is frequently used as a direct substitute for the relative particle sem (meaning who, which, or that). They function identically in this context, but using er elevates the tone of the text to a more academic or literary level.

Since er can also mean "is" or "when", how can I tell it means "which" in this sentence?

You can identify its function by looking at the sentence structure. It directly follows a noun (skáldaverkið) and introduces a clause describing that noun (hún ritaði). If it meant "is" or "when" here, the grammar and meaning of the sentence would completely break down.

Why does the sentence use ritaði instead of the much more common skrifaði?

The verb rita is a formal, literary synonym for skrifa (to write). Because the sentence is discussing a literary work (skáldaverk) and already establishes a formal tone with the relative particle er, the verb ritaði perfectly matches the elevated style of the sentence.

What determines the case of skáldaverkið? Does the relative particle er play a role?