Breakdown of Hún heldur að hann skynji hugarfarið hjá sér.
hún
she
hann
he
halda
to think (believe)
hjá
next to / at (someone's) place, with
Questions & Answers about Hún heldur að hann skynji hugarfarið hjá sér.
Why does sér mean "her" here instead of using hennar, considering the subject of that clause is hann?
This is long-distance reflexivization in action! Because the main verb is a verb of thinking (heldur) and the subordinate verb is in the subjunctive mood (skynji), the reflexive pronoun sér can "reach across" the clause boundary and refer all the way back to the subject of the main clause, hún.
Could sér also refer to hann in this sentence?
Grammatically, yes. Sér normally reflects back to the subject of its own clause, so it could mean "he perceives his own mindset." However, when a sentence uses a subjunctive "thinking" verb, native speakers naturally interpret the reflexive as pointing back to the main thinker (hún). To unambiguously say "his mindset," you would normally use hugarfarið hans.
Why is the verb skynja (to perceive) in the subjunctive form skynji?
It is triggered by the main verb halda (to think/believe). When you express a subjective belief, thought, or reported speech in a subordinate clause introduced by að, the verb takes the subjunctive mood. This subjunctive mood is the essential "bridge" that allows the long-distance reflexive sér to work.
Why does the sentence use hjá sér instead of just a possessive pronoun to mean "her mindset"?