Það var uppselt í stóra salnum, svo sumir stóðu aftast.

Here það is a dummy subject. It does not point to a specific thing.

In Icelandic, expressions like það var uppselt are very common and work much like English it was sold out or there was no room left. So það is needed grammatically, even though it does not really mean a concrete it.

Because this is an impersonal construction: það var uppselt.

When Icelandic uses a dummy subject like það, the predicate often appears in the neuter singular, which is why you get uppselt.

So:

• uppseldur = masculine singular
• uppseld = feminine singular
• uppselt = neuter singular

Here the form is not agreeing with salnum. It is just the default neuter singular form used in this kind of expression.

Uppselt is the past participle of uppselja / selja upp, used adjectivally, and it means sold out.

So in það var uppselt, it behaves like a predicate adjective:

• það var uppselt = it was sold out

Learners often find this confusing because it looks verbal and adjectival at the same time. In practice, you can think of it as an adjective meaning sold out.

Because í takes the dative when it means in a place, with no movement involved.

Here the meaning is location:

• í stóra salnum = in the big hall

So the noun phrase is in the dative.

A very useful contrast is:

• í salnum = in the hall → location, so dative
• í salinn = into the hall → movement toward, so accusative

Because the noun phrase is definite and in the dative singular.

In Icelandic, when a noun has the definite article, the adjective usually takes the weak form. That is why you get:

• stóri salurinn = the big hall
• í stóra salnum = in the big hall

So stóra is the weak adjective form used with a definite noun.

Because Icelandic usually puts the definite article as a suffix on the noun.

So instead of a separate word like English the, Icelandic often does this:

• salur = hall
• salurinn = the hall
• salnum = the hall in the dative singular

This is one of the most important structural differences from English.

The dictionary form is salur, meaning hall.

In the sentence, it appears as salnum because it is:

• dative singular
• definite

So when you look it up, you would look for salur, not salnum.

Here svo means so in the sense of therefore / as a result.

So the sentence structure is:

• Það var uppselt í stóra salnum
• svo sumir stóðu aftast

= It was sold out in the big hall, so some stood at the back.

Be careful: svo can mean different things in different contexts, such as so, then, or sometimes something closer to very in informal speech. Here it is clearly the conjunction so.

Because Icelandic can use sumir by itself to mean some people or some of them when the noun is understood from context.

So:

• sumir = some
• implied meaning here: some people / some audience members

This is very natural in Icelandic.

Also, sumir is plural nominative, because it is the subject of the clause.

Sumir is the masculine plural nominative form.

When Icelandic refers to a mixed group of people, or people in general, the masculine plural is often used as the default. That is why sumir works here.

If the group were specifically feminine, you might see sumar instead.

Stóðu is the past tense plural of standa (to stand).

Because the subject is plural:

• sumir = some people
• the verb must also be plural
• so: sumir stóðu = some stood

A useful comparison:

• hann stóð = he stood
• þeir stóðu = they stood

Aftast is an adverb, meaning furthest back / at the very back.

It is an adverb because it describes where they stood, not what kind of people they were.

So:

• sumir stóðu aftast = some stood at the back

You can think of it as part of a position scale:

• aftar = further back
• aftast = furthest back / at the very back

If it were an adjective, it would have to describe a noun directly, which it does not do here.

Yes. Icelandic allows some flexibility in word order, especially to shift emphasis.

For example, you could also say:

• Í stóra salnum var uppselt, svo sumir stóðu aftast.

This still means the same thing, but it puts more focus on in the big hall at the beginning.

The original sentence is very natural and neutral, though.