Questions & Answers about Frá hverju kemur áfallið sem þú manst?
Why do we use hverju here instead of hvað for "what"?
The preposition frá (from) always triggers the dative case. While the dictionary form of "what" is hvað, its dative form is hverju. Therefore, "from what" translates to frá hverju.
The verb muna (to remember) becomes manst here. Why does it look so different from its infinitive?
Muna belongs to a small group of irregular verbs (called preterite-present verbs) where the vowel changes in the present tense. For þú (you), the correct present tense form is manst. These don't follow the regular verb patterns and just have to be memorized.
In English, we can just say "the trauma you remember" and drop the word "that". Can we drop sem in Icelandic?
No, you cannot drop sem. While English allows us to omit the relative pronoun "that" or "which" when it isn't the subject of the clause, Icelandic strictly requires sem to link the two parts of the sentence together.
Why is the definite article -ið attached to áfall?
Áfall (trauma or shock) is a neuter noun. Because it acts as the subject of the verb kemur, it is in the nominative case. The nominative definite article suffix for neuter nouns is -ið, making it áfallið.