Millifærslan fer ekki í gegn ef reikningsnúmerið er rangt.

Millifærslan means the transfer.

The base noun is millifærsla = transfer (especially a bank transfer).
The ending -n is the definite article added to the noun, so:

• millifærsla = a transfer
• millifærslan = the transfer

In Icelandic, the is usually attached to the end of the noun instead of being a separate word.

This is an idiomatic expression. Literally, the parts are:

• fer = goes
• ekki = not
• í gegn = through

But together, fara í gegn often means to go through, to be accepted, or to be processed successfully.

So here:

• Millifærslan fer ekki í gegn = The transfer does not go through

This is very similar to English banking language.

Because in a normal Icelandic main clause, the finite verb usually comes early, and ekki commonly comes right after it.

So:

• Millifærslan fer ekki í gegn

breaks down as:

• Millifærslan = subject
• fer = finite verb
• ekki = not
• í gegn = particle/prepositional part of the expression

This placement of ekki is very common in Icelandic.

Compare:

• Hann kemur ekki. = He is not coming.
• Þetta virkar ekki. = This does not work.

Icelandic often uses the present tense where English would use will.

So:

• Millifærslan fer ekki í gegn ef reikningsnúmerið er rangt

is literally present tense, but naturally means:

• The transfer will not go through if the account number is wrong

This is very normal in Icelandic, especially in general statements, instructions, and conditions.

Ef means if.

It introduces a conditional clause:

• ef reikningsnúmerið er rangt = if the account number is wrong

This works much like English if.

In this sentence, the ef-clause explains the condition under which the transfer will not go through.

Reikningsnúmerið means the account number.

It is made up of:

• reikningur = account
• númer = number

In compounds, the first part often changes form slightly:

• reiknings-
  • númer → reikningsnúmer

Then the definite article is added:

• reikningsnúmer = an account number
• reikningsnúmerið = the account number

So this is a very typical Icelandic compound noun.

Because reikningsnúmer is a neuter noun, and -ið is the definite ending here.

So:

• reikningsnúmer = account number
• reikningsnúmerið = the account number

This is one of the common neuter definite endings in Icelandic.

Because the adjective has to agree with the noun it describes.

Here the noun is:

• reikningsnúmerið = the account number

That noun is neuter singular, so the adjective must also be neuter singular:

• rangur = masculine
• röng = feminine
• rangt = neuter

Since reikningsnúmerið is neuter, Icelandic uses:

• reikningsnúmerið er rangt = the account number is wrong

Because this is a standard be + adjective structure.

• reikningsnúmerið er rangt
• literally: the account number is wrong

Just like in English, the noun comes first, then the form of to be (er), then the adjective.

They are similar, and in many everyday situations both could work, but there is a slight difference in feel.

• rangt = wrong / incorrect
• vitlaust = wrong / mistaken / faulty / silly, depending on context

In a formal or technical context like banking, rangt often sounds especially natural because it clearly means incorrect.

So:

• reikningsnúmerið er rangt = the account number is incorrect

That fits this sentence very well.

The structure is:

• Millifærslan = subject
• fer ekki í gegn = main verb phrase
• ef reikningsnúmerið er rangt = conditional clause

So the full pattern is:

• [Main clause] + ef + [condition]

This is directly comparable to English:

• The transfer will not go through if the account number is wrong.

Yes. Icelandic can also put the condition first:

• Ef reikningsnúmerið er rangt, fer millifærslan ekki í gegn.

That means the same thing:

• If the account number is wrong, the transfer will not go through.

Notice that when the ef-clause comes first, the verb in the main clause still comes early:

• ... fer millifærslan ekki í gegn

That is part of Icelandic verb-second-style word order.

A rough learner-friendly pronunciation guide would be:

• Millifærslan fer ekki í gegn ef reikningsnúmerið er rangt
• roughly: MID-li-fairst-lan fer EHK-ki ee GYAIN ef RAYK-nings-noo-meh-rith er rangkt

A few useful notes:

• ll in Icelandic often does not sound like normal English l
• æ sounds somewhat like the vowel in my or eye, depending on the word
• þ and ð do not appear here, but g and gn clusters can sound unusual to English speakers
• reiknings- may feel long because of the compound structure

If you are learning pronunciation, it helps to listen to native audio because Icelandic spelling is regular, but some sounds are quite different from English.

Yes, it sounds natural, especially in banking, apps, websites, or customer-service language.

It has a clear, practical style:

• Millifærslan fer ekki í gegn ef reikningsnúmerið er rangt.

This is exactly the kind of sentence you might see in an online banking system or an error/help message.