Learn Icelandic: Góðir borðsiðir krefjast þess að þú notir gaffalinn. - Good table manners require that you use the fork.

Notir is the present subjunctive form of the Group 1 weak verb nota (to use). The verb krefjast (to require) expresses a demand, which triggers the subjunctive mood in the clause that follows it. For Group 1 verbs, the subjunctive is formed by changing the vowel in the ending to -i, making the þú form notir instead of the regular indicative notar.

Why do we need the word þess before að?

The verb krefjast requires its object to be in the genitive case. Because the object here is an entire sub-clause starting with að (that), Icelandic grammar requires us to insert the pronoun það (it) in its genitive form, þess, to satisfy the verb's case rule. You can think of it as saying that good manners require of it, that you use the fork.

Why does the verb krefjast end in -st?

This is the middle voice (miðmynd) form of the verb. While the active verb krefja exists, the middle voice krefjast is the standard way to express to require or to demand in the sense of a rule or condition demanding something.

Why does the word for the fork only have one l in the middle of gaffalinn?