Á reikningnum stóð sautján þúsund krónur, en hún hélt að það væru bara sextán.

It can mean any of those depending on context.

In this sentence, á reikningnum stóð ... most naturally means something like the bill/invoice said/listed .... If the wider context were banking, it could also mean on the account statement or in the account.

So the exact English word depends on the situation, but the Icelandic word itself is broader than just one English equivalent.

Because á here means on in the sense of location, and with that meaning it takes the dative case.

So:

• reikningur = an account / a bill
• reikningnum = the account / the bill, in dative singular

The ending -num is the dative singular definite ending here.

So á reikningnum means on the bill/account.

Yes, literally stóð is the past tense of standa = to stand.

But Icelandic often uses standa the way English uses say, be written, or be listed.

So:

• Á reikningnum stóð sautján þúsund krónur
  = literally, On the bill stood seventeen thousand krónur
• natural English: The bill said/listed seventeen thousand krónur

This is a very common idiomatic use.

This is because Icelandic is a verb-second language.

That means the finite verb normally comes in the second position of a main clause. Here, the sentence starts with Á reikningnum to set the scene, so the verb stóð has to come next.

Structure:

• Á reikningnum = first element
• stóð = finite verb in second position
• sautján þúsund krónur = the amount

So the order is natural Icelandic word order, not an inversion for emphasis in the English sense.

Because a sum of money is often treated as one total amount rather than as many separate coins or units.

In this kind of sentence, Icelandic commonly uses singular agreement with amounts, especially when the point is the figure written on the bill.

So sautján þúsund krónur is understood as one sum: 17,000 krónur.

That is why singular stóð sounds natural here.

Because the amount is more than one króna, so the noun is plural.

• króna = one króna
• krónur = krónur, plural

So:

• sautján þúsund krónur = seventeen thousand krónur

This works much like English seventeen thousand dollars.

Because væru is the past subjunctive of vera.

After verbs like halda að = to think / believe that, Icelandic often uses the subjunctive when the thought is being reported as uncertain, mistaken, or contrary to reality from the speaker’s point of view.

So:

• hún hélt að ... væru ...
  = she thought that ... were ...

In this sentence, she turned out to be wrong, so the subjunctive fits very well.

A useful pattern to remember is:

• Ég hélt að hann væri heima
  = I thought he was at home

Because það here is a dummy subject. It does not control the agreement in the usual way.

The verb agrees with the real idea behind the clause, which is the omitted amount:

• sextán [þúsund krónur]

Since þúsund krónur is plural in form, the verb is plural too:

• það væru bara sextán
  = that it was only sixteen [thousand krónur]

This is similar to other Icelandic sentences like:

• Það eru margir hér
  = There are many here

Even though það is singular in form, the verb can be plural because það is just filling the subject slot.

It is simply omitted because it is already understood from the previous clause.

The full meaning is:

• ... hún hélt að það væru bara sextán þúsund krónur

But Icelandic, like English, often leaves out repeated words when they are obvious from context.

Compare English:

• The bill said 17,000, but she thought it was only 16.

You do not need to repeat thousand or krónur if the listener already knows what number is being discussed.

Bara means only, just, or sometimes simply, depending on context.

Here it means only:

• bara sextán = only sixteen

So it shows that she thought the amount was lower than it really was.

In this sentence, bara adds the idea of merely or no more than.