Learn Icelandic: Hún elskar hann af allri sálu sinni.

Even though sinni is often translated as 'her', it is a reflexive pronoun that refers back to the subject of the sentence (hún). If the sentence used hennar, it would mean she loves him with some other woman's soul. Since it's her own soul, we must use the reflexive sinni to match the subject.

Why is the preposition af used here instead of með to mean 'with'?

While með translates to 'with' when talking about tools or accompaniment, Icelandic typically uses af for idiomatic expressions involving deep emotions, origins, or states of mind. You will also see this in similar phrases like af öllu hjarta (with all one's heart).

What case is the phrase allri sálu sinni in, and what causes it?

The entire phrase is in the dative case because it follows the preposition af, which always governs the dative. The words allri (all), sálu (soul), and sinni (her) all agree in the feminine dative singular form.

Why is the word for soul sálu here instead of sál?

The noun for 'soul' has two valid variations in Icelandic: the strong feminine sál and the weak feminine sála. In the dative case, sál remains sál, but sála becomes sálu. The weak form sálu is heavily preferred in fixed, emotional expressions like this one.