Ef pöntunin kemur ekki í dag, verða það mikil vonbrigði fyrir okkur bæði.

Pöntun means order. The ending -in is the suffixed definite article, so pöntunin means the order.

Here it is the subject of the clause, so it appears in the nominative singular. The noun pöntun is feminine.

Literally, kemur is the 3rd-person singular present tense of koma, which basically means comes.

But with things like orders, packages, letters, buses, and so on, Icelandic koma is very often used where English would naturally say arrive. So:

• Pöntunin kemur í dag = The order arrives/is coming today

That is very normal in Icelandic. Icelandic often uses the present tense for future events, especially in clauses with words like ef (if) or with time expressions.

So Ef pöntunin kemur ekki í dag is literally If the order does not come today, but in natural English it is often best translated as If the order doesn’t arrive today.

In a neutral Icelandic clause, ekki usually comes after the finite verb.

So:

• pöntunin kemur ekki = the order does not arrive

This is the normal word order:

• subject + finite verb + ekki

Because Icelandic is a verb-second language in main clauses.

The whole first clause Ef pöntunin kemur ekki í dag counts as the first element. After that, the finite verb of the main clause must come next, so you get:

• Ef pöntunin kemur ekki í dag, verða það ...

If the main clause stood alone, the order would normally be:

• Það verða mikil vonbrigði fyrir okkur bæði.

So the fronted if-clause causes the verb to move in front of það.

Not exactly. Here það works a lot like English it in a sentence such as It will be a disappointment.

It refers more to the whole situation than to the noun pöntunin by itself. In other words, the meaning is closer to:

• If the order doesn’t arrive today, that / it will be a great disappointment...

So það is not simply repeating the order.

Because vonbrigði is a plural-only noun in Icelandic. Even when English says a disappointment in the singular, Icelandic normally uses vonbrigði, which is grammatically plural.

That affects the grammar around it:

• verða = plural present form
• mikil = plural neuter form of mikill

So:

• mikil vonbrigði = literally great disappointments
• but naturally in English: a great disappointment

This is one of those places where Icelandic and English package the idea differently.

Yes, verða often means become. But in sentences like this, it can also mean something like turn out to be or simply be, especially when English would naturally use will be.

So here:

• verða það mikil vonbrigði
  means something like
• that will turn out to be a great disappointment

In natural English, will be is the best translation.

Fyrir okkur bæði means for both of us.

• okkur = us
• bæði = both

In Icelandic, bæði often comes after a pronoun in this kind of expression:

• við bæði = we both
• okkur bæði = both of us

So the word order here is perfectly normal.

Because the sentence begins with a subordinate if-clause:

• Ef pöntunin kemur ekki í dag, ...

In standard Icelandic writing, an introductory subordinate clause is commonly separated from the main clause with a comma. So the comma here is normal punctuation.