Hurðin er hreinni núna.

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

• hurð = door
• hurðin = the door

So the -in at the end is the definite article here.

Hurðin is nominative singular definite.

More specifically:

• the noun is hurð
• it is a feminine noun
• here it is the subject of the sentence, so it is in the nominative
• the ending makes it definite: the door

So this form means the door as the subject of the sentence.

Er is the 3rd person singular present tense of vera (to be).

Since hurðin is a singular subject, Icelandic uses the singular verb:

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

So Hurðin er ... means The door is ...

Because the sentence uses the comparative form of hreinn (clean).

So:

• hreinn / hrein / hreint = clean
• hreinni = cleaner in the form needed here

Icelandic usually expresses cleaner with a changed adjective form, not with a separate word like more clean.

Dictionaries usually list Icelandic adjectives in the masculine nominative singular form. For this adjective, that form is hreinn.

But in an actual sentence, the adjective has to match the noun it describes.

Since hurð is feminine singular, the adjective does not stay in the dictionary form.

For example:

• Hurðin er hrein. = The door is clean.
• Hurðin er hreinni. = The door is cleaner.

So hreinn is the base form you look up, but it is not the form you use here.

Yes. In Icelandic, predicate adjectives after vera (to be) normally agree with the subject in gender, number, and case.

Here:

• hurðin is feminine singular nominative
• hreinni is the comparative form that matches that subject

That agreement is one of the big differences from English, where clean and cleaner do not change according to grammatical gender.

Yes, in a sense. A comparative like hreinni usually implies some comparison, even if Icelandic does not say it explicitly.

So Hurðin er hreinni núna naturally means something like:

• the door is cleaner than before
• the door is cleaner now than it was

If you want to say the comparison explicitly, Icelandic can add en (than), for example hreinni en áður (cleaner than before).

Núna means now and tells you the time frame of the statement.

So it marks a change or current state: the door is cleaner now.

You will also see nú, which can often mean the same thing. In many everyday contexts, nú and núna are very close, though núna often feels a bit more conversational or time-specific.

Yes. Icelandic word order is somewhat flexible, and emphasis can change the order.

The given sentence is the neutral order:

• Hurðin er hreinni núna.

But you could also say:

• Núna er hurðin hreinni.

That puts more emphasis on now. Because Icelandic is a verb-second language, when you move núna to the front, the verb er still comes second.

So both are natural, but the focus is slightly different.