Boken er tynn.

Because boken is the definite singular form: the book.

• bok = a book
• boken = the book

In Norwegian, definiteness is often shown by adding an ending to the noun instead of using a separate word like the in English.

Here, -en marks the noun as definite singular.

So:

• en bok = a book
• boken = the book

This is a very common Norwegian pattern. English uses a separate article (the), but Norwegian often attaches the definiteness directly to the noun.

Yes, in Bokmål, boka er tynn is also correct.

The noun bok is traditionally feminine, so you may see:

• boka = the book (feminine form)

But in Bokmål, many feminine nouns can also take the common-gender/masculine-style definite ending:

• boken = also the book

So both are possible in Bokmål:

• Boka er tynn
• Boken er tynn

Which one sounds more natural can depend on style, dialect, and personal preference.

Because tynn agrees with the noun it describes.

Here, bok/boken is a common-gender singular noun, so the adjective stays in its basic form:

• en tynn bok = a thin book
• Boken er tynn = The book is thin

Compare:

• et tynt ark = a thin sheet → tynt for neuter singular
• bøkene er tynne = the books are thin → tynne for plural

So in this sentence, tynn is the correct form.

Because tynn is used after the verb er. This is called predicative use.

Compare:

• Den tynne boken = the thin book

  • adjective comes before the noun
  • definite phrase
  • adjective becomes tynne

• Boken er tynn = The book is thin

  • adjective comes after er
  • adjective is predicative
  • adjective is tynn

So even though boken is definite, the adjective does not become tynne here.

Er means is / are / am, depending on context. It is the present tense of å være (to be).

Examples:

• Jeg er trøtt = I am tired
• Hun er glad = She is happy
• Boken er tynn = The book is thin

Norwegian uses the same present-tense form er for all persons.

This is the normal basic word order for a simple statement:

subject + verb + adjective

• Boken = subject
• er = verb
• tynn = adjective/complement

So Boken er tynn follows the same basic pattern as English The book is thin.

You usually put ikke after the verb:

• Boken er ikke tynn = The book is not thin

That is the normal position in a simple sentence like this.

You switch the verb and subject:

• Er boken tynn? = Is the book thin?

This is a very common Norwegian pattern for yes/no questions.

The tricky part is the vowel y, which does not exist in the same way in standard English.

A rough guide:

• tynn sounds approximately like tünn
• the y is pronounced with rounded lips

A useful tip:

1. Start by saying ee as in see
2. Keep your tongue in roughly that position
3. Round your lips

That gets you closer to Norwegian y.

Also, tynn has a short vowel and a double n, so it sounds short and crisp.

Yes, it is grammatically natural.

Whether it is natural in context depends on what kind of book you mean:

• If you mean a physically thin book, booklet, or slim volume, it sounds fine.
• In some situations, Norwegian speakers might also choose words like slank or just describe the number of pages, depending on context.

But as a basic learning sentence showing structure and adjective agreement, Boken er tynn is completely normal.