Learn Hardcore Icelandic: 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.

What exactly is sammála — is it a verb or an adjective, and how do I use it?

Sammála is (functionally) an adjective meaning of the same opinion, used with vera (to be). It corresponds to English to be in agreement / to agree.

Basic pattern:

Ég er sammála. – I agree.
Við erum sammála. – We agree.

With the person you agree with, you use the dative case, and no preposition:

Ég er sammála þér. – I agree with you.
Við erum sammála kennaranum. – We agree with the teacher.

With the topic you agree about, you normally add um (about). That’s what happens in your sentence:

Við erum sammála um að þetta partí sé skemmtilegt.
We agree (are of the same opinion) about the fact that this party is fun.

So Icelandic does not have a separate simple verb “to agree” here; instead it uses vera + sammála.

Why is the preposition um used after sammála?

Um means about, concerning. After sammála, it marks the topic of the agreement.

Two main patterns:

Agreeing with a person → dative, no um
- Ég er sammála þér. – I agree with you.
- Við erum sammála honum. – We agree with him.
Agreeing about something (a topic, fact, or clause) → um
- Við erum sammála um þetta mál. – We agree about this issue.
- Við erum sammála um að þetta partí sé skemmtilegt. – We agree that this party is fun.

You might hear some speakers say sammála að… without um, but sammála um að… is the clearer and more standard pattern.

A slightly more formal alternative is:

Við erum sammála því að þetta partí sé skemmtilegt.
(literally: we are agreed on it that this party is fun)

What is the role of að in um að þetta partí sé skemmtilegt?

Here að is a conjunction meaning that, introducing a subordinate clause. It links sammála um to a whole sentence: þetta partí sé skemmtilegt.

So:

um að þetta partí sé skemmtilegt
≈ about [the fact] that this party is fun

It is not the infinitive að (as in að borða – to eat); instead, it is the “that” which English often can drop:

English can say: We agree that this party is fun or We agree this party is fun.
Icelandic basically always needs að here:
Við erum sammála um að þetta partí sé skemmtilegt.

Why is the adjective skemmtilegt (neuter) and not skemmtileg or skemmtilegur?

Because adjectives in Icelandic must agree in gender, number, and case with the noun they describe. Partí is a neuter singular noun in the nominative, so the adjective must be neuter singular nominative: skemmtilegt.

Relevant forms of skemmtilegur (fun, entertaining):

Masculine nom. sg.: skemmtilegur – skemmtilegur bíll (a fun car)
Feminine nom. sg.: skemmtileg – skemmtileg bók (a fun book)
Neuter nom. sg.: skemmtilegt – skemmtilegt partí (a fun party)

In the clause að þetta partí sé skemmtilegt, the subject is þetta partí (neuter singular), so the predicative adjective must be skemmtilegt.

Does sammála change form for gender or number?

No. Sammála is effectively indeclinable in modern usage: it has the same form regardless of gender, number, or case of the subject.

You say:

Ég er sammála. – I agree.
Hann er sammála. – He agrees.
Hún er sammála. – She agrees.
Þau eru sammála. – They agree.
Við erum sammála. – We agree.

It is always sammála; you don’t get forms like sammálar, sammálarir, etc.

What does change is:

the form of vera (er, ertu, erum, eru, …)
the case of the person you agree with: Ég er sammála þér / honum / þeim (all dative).

Could the clause að þetta partí sé skemmtilegt be phrased differently?

Yes, there are a few natural variations that keep the same basic meaning. For example:

Við erum sammála um að þetta sé skemmtilegt partí.
– We agree that this is a fun party.
Here skemmtilegt becomes an attributive adjective in front of partí.
Við erum sammála um að partíið sé skemmtilegt.
– We agree that the party is fun.
Now you have the definite form partíið (the party).

Things that are not standard in careful Icelandic:

✗ Við erum sammála um þetta partí sé skemmtilegt.
(missing að before the clause)
✗ Við erum sammála um að þetta partí er skemmtilegt.
(using indicative er instead of subjunctive sé after sammála um að)

Both are understandable and might be heard in casual speech, but the original form with að and sé is preferable.

Why is the word partí used, and are there more “Icelandic-sounding” words for party?

Partí is a very common modern loanword from English party and is perfectly normal in everyday Icelandic, especially for informal social gatherings.

There are also more traditional/native words:

veisla (feminine): a feast, banquet, or more formal party
- Þetta er skemmtileg veisla. – This is a fun feast/party.
boð (neuter): a party or an invitation/gathering
- Þetta er skemmtilegt boð. – This is a fun gathering/party.

If you replace partí with veisla in your sentence, the adjective must change to feminine:

Við erum sammála um að þessi veisla sé skemmtileg.
(this feast/party is fun)

How would I say “We both agree with you that this party is fun” using the same structure?

You can say:

Við erum bæði sammála þér um að þetta partí sé skemmtilegt.

Breakdown:

Við – we
erum – are
bæði – both (refers to við)
sammála þér – agree with you (dative þér)
um að þetta partí sé skemmtilegt – about the fact that this party is fun.

This keeps the same grammar as the original sentence and just adds bæði (both) and þér (you, dative).