Learn Icelandic: Skyrtan er saumuð úr silki. - The shirt is sewn out of silk.

Because in the static passive, the past participle acts like an adjective and has to agree with the subject. The subject here is skyrtan (the shirt), which is a feminine noun in the nominative case. Therefore, the past participle of sauma (to sew) takes the feminine singular nominative form, which is saumuð.

What makes this a 'static passive' sentence?

A static passive describes a state that is the result of a past action, rather than an action happening right now. It is formed using the verb vera (to be) plus a past participle. Here, er saumuð tells us the current state of the shirt (it is sewn out of silk), not that someone is in the middle of sewing it right this moment.

Why is the word silki in the dative case here?

The preposition úr (out of, from) always forces the noun that follows it to take the dative case. Even though the word silki looks exactly the same in the nominative, accusative, and dative cases, it is grammatically dative here because it comes right after úr.

How can I tell that skyrtan means 'the shirt' rather than 'a shirt'?