Learn Icelandic: Samdir þú laglínuna? - Did you write the melody?

While skrifa means to physically write text like a letter or a book, semja means to compose or create. You use semja for writing music, poetry, essays, or even drafting contracts.

How does the verb semja conjugate to samdir here?

Semja has an irregular past tense. The root vowel shifts from 'e' to 'a', and the 'j' drops out, giving the past stem samd-. For the pronoun þú, you add the ending -ir, making it samdir.

How does the grammar change if I want to say 'Did you write YOUR melody?' to fit this lesson's theme?

To ask neutrally, place the possessive pronoun after the definite noun, which gives you Samdir þú laglínuna þína?. But to put focus on the possessive (meaning YOUR melody, not someone else's), move the pronoun before the noun and drop the definite article, which gives you Samdir þú þína laglínu?.

Why is laglínuna in the accusative case?

It is the direct object of the verb semja. The compound noun laglína (made from lag meaning tune, and lína meaning line) is feminine, so its definite article becomes -na in the accusative case.