Það voru mikil vonbrigði að missa af leiknum.

Here það is mostly a dummy subject, much like English it in sentences such as It was disappointing to miss the game.

It does not really refer to a specific thing. Icelandic often uses það this way when the real content of the sentence comes later, here in að missa af leiknum.

So in this sentence, það is there because Icelandic wants a subject slot filled, not because that/it has a strong concrete meaning.

Because vonbrigði is grammatically plural, and the verb agrees with that.

• voru = were
• var = was

Even though English often says it was a disappointment, Icelandic uses the noun vonbrigði, which is treated as plural in form. So the sentence uses plural agreement:

• Það voru mikil vonbrigði ...
• literally something like It/there were great disappointments ...

That sounds strange in English, but it is normal Icelandic grammar.

Because mikil agrees with vonbrigði.

The noun vonbrigði is:

• neuter
• plural

So the adjective must also be neuter plural. That gives:

• mikill = masculine singular
• mikil = neuter plural / feminine singular / some other forms depending on context

Here, mikil is the correct agreement form for vonbrigði.

It is a noun that is normally used in the plural.

So even when the meaning in English is something like a disappointment or great disappointment, Icelandic uses vonbrigði as a plural-form noun.

That is why you get:

• mikil vonbrigði
• voru rather than var

This is one of those places where you should trust Icelandic grammar rather than try to match English word-for-word.

Að missa af is the expression for to miss in the sense of fail to catch, fail to attend, or miss out on something.

So:

• missa af leiknum = miss the game
• missa af strætónum = miss the bus
• missa af tækifærinu = miss the opportunity

The preposition af is part of the expression. You should learn missa af as a unit.

Because missa and missa af are not the same.

• missa af leiknum = miss the game (not be there, not see it, not catch it)
• missa leikinn would more likely be understood differently, often closer to lose the game

So for events, transport, chances, and similar things that you fail to catch or attend, Icelandic usually uses missa af + dative.

That is a very common learner mistake, because English just says miss the game without a preposition.

Because af takes the dative case, and leiknum is the dative singular definite form of leikur.

The forms are:

• leikur = a game, a match
• leiknum = the game, the match (dative singular definite)

So:

• af leiknum = of/from the game grammatically, but in the phrase missa af leiknum, it means miss the game

This is a good example of why case matters in Icelandic: the preposition controls the form of the noun.

Here að is the infinitive marker, like English to.

So:

• að missa = to miss

In this sentence, the whole infinitive phrase is:

• að missa af leiknum = to miss the game

That phrase acts as the real content of the sentence: what was disappointing was missing the game.

Yes. This is a very normal Icelandic structure.

Icelandic often says:

• Það + vera + noun/adjective + að + infinitive

So this sentence is built like:

• Það = dummy subject
• voru = were
• mikil vonbrigði = a great disappointment
• að missa af leiknum = to miss the game

This is similar to English patterns like:

• It was disappointing to miss the game
• It was a great disappointment to miss the game

So although the grammar details are different, the overall structure is quite natural.

It is in the nominative, because it is the predicate noun after vera (to be).

In a sentence with vera, the noun describing what something is normally appears in the nominative:

• Þetta eru vonbrigði = This is a disappointment / These are disappointments

So in Það voru mikil vonbrigði ..., both mikil and vonbrigði are in the form required for a nominative neuter plural predicate.