Questions & Answers about Ég held að íslenskan sé erfið, en það er gaman.
Að is the conjunction that, used to introduce a subordinate clause after verbs like halda (to think / believe), segja (to say), vita (to know), etc.
So the structure is basically: Ég held [að + clause].
Sé is the present subjunctive form of vera (to be). Icelandic often uses the subjunctive after að with verbs of opinion/reporting like Ég held..., especially when the statement is framed as a thought, claim, or subjective assessment rather than a plain fact.
You may also see Ég held að íslenskan er erfið, using the indicative er, which can sound more like you’re stating it as a straightforward fact. The subjunctive (sé) is very common and often feels more natural in careful standard Icelandic in this pattern.