Questions & Answers about Við erum sammála um að þetta partí sé skemmtilegt.
Sé is the present subjunctive form of vera (to be). Icelandic often uses the subjunctive in subordinate clauses that express opinions, beliefs, or evaluations, rather than hard facts.
Við erum sammála um að þetta partí sé skemmtilegt.
≈ We agree that this party is fun.
Because vera sammála um að… expresses an opinion (not certain knowledge), the clause að þetta partí sé skemmtilegt normally takes the subjunctive sé.
Compare:
- Ég held að hann sé veikur. – I think he is sick. (opinion → subjunctive)
- Ég veit að hann er veikur. – I know he is sick. (fact/knowledge → indicative)
You will hear …að þetta partí er skemmtilegt in everyday speech, but sé is the form taught as standard and sounds more natural in careful Icelandic here.