Bordduken er ren.

Because bordduken is the definite singular form: the tablecloth.

In Norwegian, the definite article is usually added to the end of the noun instead of being a separate word in front.

• en bordduk = a tablecloth
• bordduken = the tablecloth

So the sentence uses bordduken because it is talking about a specific tablecloth.

Norwegian often makes compound nouns by joining words together.

Here:

• bord = table
• duk = cloth

So bordduk literally means table-cloth, written as one word. This is very normal in Norwegian.

Bordduk is normally a masculine/common-gender noun in Bokmål.

That gives you:

• en bordduk = a tablecloth
• bordduken = the tablecloth

For a learner, the important thing is to memorize it with its article: en bordduk.

Er is the present tense of å være (to be).

So:

• jeg er = I am
• du er = you are
• bordduken er = the tablecloth is

Unlike English, Norwegian does not change the present-tense form of the verb depending on the subject. It is always er in the present.

Because adjectives in Norwegian often agree with the noun in gender and number, even when they come after er.

Here, bordduken is:

• singular
• common gender

So the adjective is:

• ren = common-gender singular

Compare:

• et bord er rent = a table is clean
  (rent for neuter singular)
• bordene er rene = the tables are clean
  (rene for plural)

So Bordduken er ren uses ren because bordduken is singular and common gender.

No. In this sentence, ren is a predicative adjective because it comes after er.

Predicative adjectives usually agree in gender and number, but not in the same way as adjectives placed directly before a noun.

Compare:

• den rene bordduken = the clean tablecloth
  Here the adjective is before the noun, so you get the definite-style form rene.
• bordduken er ren = the tablecloth is clean
  Here the adjective is after er, so you use ren.

That difference is very important in Norwegian.

It is a descriptive adjective meaning clean in this context.

It can also mean things like pure in other contexts, depending on what is being described. But with bordduk, the natural meaning is clean.

This is the normal word order for a simple statement:

• subject + verb + adjective/complement

So:

• Bordduken = subject
• er = verb
• ren = complement

This is similar to English: The tablecloth is clean.

Yes, if the tablecloth is already clear from context.

• Bordduken er ren. = The tablecloth is clean.
• Den er ren. = It is clean.

Since bordduk is a common-gender noun, the pronoun is den, not det.

It depends on the noun’s gender.

• den is used for common-gender nouns
• det is used for neuter nouns

Since bordduk is a common-gender noun, you would say:

• Bordduken er ren. Den er ren.

But with a neuter noun, for example et bord (a table), you would say:

• Bordet er rent. Det er rent.

Yes. It is a completely natural and correct sentence.

It is short and simple, but exactly the kind of sentence Norwegian speakers would use when describing something:

• Skjorten er ren. = The shirt is clean.
• Gulvet er rent. = The floor is clean.
• Bordduken er ren. = The tablecloth is clean.

A learner-friendly approximation is:

• BORD-doo-ken

A few useful notes:

• bord has a long vowel sound somewhat like boor in careful pronunciation
• duk sounds roughly like dook
• the ending -en is the definite ending

The exact pronunciation varies a bit by region, but the important thing is to recognize that bordduken is one word with the definite ending attached.