Númeraplatan er hrein.

Because Icelandic usually puts the definite article on the end of the noun instead of using a separate word.

• númeraplata = a license plate / number plate
• númeraplatan = the license plate / the number plate

So the -n at the end is doing the job that the does in English.

Icelandic usually writes compound nouns as a single word.

So:

• númera
  • plata → númeraplata

This is very normal in Icelandic. English often uses two words where Icelandic prefers one compound word.

It means the license plate or the number plate.

In everyday English, the most natural translation depends on the variety of English:

• British English: number plate
• American English: license plate

So the Icelandic sentence is talking about the plate on a vehicle.

Er is the 3rd person singular present tense of að vera, meaning to be.

So here it means is.

A few present-tense forms are:

• ég er = I am
• þú ert = you are
• hann / hún / það er = he / she / it is

Because Icelandic adjectives must agree with the noun in gender, number, and usually case.

The noun númeraplata is:

• feminine
• singular
• nominative

So the adjective must also be feminine singular nominative:

• masculine: hreinn
• feminine: hrein
• neuter: hreint

That is why the sentence has hrein.

Yes. Even when an adjective comes after vera (to be), it still agrees with the noun.

So:

• Bíllinn er hreinn. = The car is clean.
  (bíllinn is masculine, so hreinn)

• Númeraplatan er hrein. = The license plate is clean.
  (númeraplatan is feminine, so hrein)

• Borðið er hreint. = The table is clean.
  (borðið is neuter, so hreint)

It is in the nominative singular because it is the subject of the sentence.

In a basic sentence with að vera, the subject is nominative, and the predicate adjective is also normally nominative.

So both parts match:

• númeraplatan — nominative singular
• hrein — nominative singular feminine

Because in this sentence hrein is not directly attached to the noun inside the noun phrase. It is a predicate adjective after the verb er.

Compare:

• hrein númeraplata = a clean license plate
  (adjective directly before the noun)

• númeraplatan er hrein = the license plate is clean
  (adjective after is)

That works much like English.

No, this is a very straightforward sentence.

It follows:

• subject: Númeraplatan
• verb: er
• adjective/complement: hrein

So it matches normal English order very closely: The license plate is clean.

A rough English-friendly approximation is hrayn.

A few notes:

• ei in Icelandic usually sounds like ay in day
• hr is a special Icelandic cluster; the h is heard, and the r is not exactly like English r

So hrein is not pronounced like English rain, but that is a useful rough starting point.

Not always. Like clean in English, hreinn / hrein / hreint can also have broader meanings such as pure or clear, depending on context.

But in Númeraplatan er hrein, the natural meaning is literal: the plate is physically clean.

Not in this sentence. In númeraplata, it is part of the compound noun.

So you should learn númeraplata as a complete vocabulary item meaning license plate / number plate, rather than trying to treat númera as a separate word in this sentence.