Boken har hundre sider.

Because boken is the definite singular form: the book.

In Norwegian, definiteness is often added as a suffix to the noun:

• en bok = a book
• boken = the book

So in Boken har hundre sider, the speaker is talking about the book, not just a book in general.

That is a normal feature of Norwegian. Instead of usually having a separate word like English the, Norwegian often attaches definiteness to the noun itself.

For example:

• en bil = a car
• bilen = the car
• ei bok / en bok = a book
• boka / boken = the book

So -en in boken works like English the.

Har is the present tense of å ha, which means to have.

So:

• å ha = to have
• har = has / have

In this sentence, Boken har hundre sider, har means has.

Norwegian verbs do not change depending on the subject the way English verbs do. So:

• jeg har = I have
• du har = you have
• boken har = the book has
• vi har = we have

It is always har in the present tense.

Because hundre is more than one, so the noun must be plural.

• en side = a page
• sider = pages

After numbers greater than one, Norwegian normally uses the plural:

• én side = one page
• to sider = two pages
• hundre sider = one hundred pages

So side would be wrong here.

A useful pattern is:

• en side = a page
• siden = the page
• sider = pages
• sidene = the pages

This is a very common noun pattern in Norwegian.

Because Norwegian does not use an article in front of plural nouns after a number.

You say:

• hundre sider = one hundred pages
• not en hundre sider

The number itself already tells you how many, so no extra indefinite article is needed.

No, this is the normal basic Norwegian word order: subject + verb + object.

• Boken = subject
• har = verb
• hundre sider = object

So the structure is very similar to English:

• The book has one hundred pages.

Yes. In Bokmål, bok is a feminine noun, so many speakers write:

• boka = the book

But Bokmål also often allows a masculine/common-gender-style form:

• boken = the book

So both of these can be correct in Bokmål:

• Boka har hundre sider
• Boken har hundre sider

Learners will often see both. Boka may sound more natural to many speakers, but boken is also standard Bokmål.

Yes, but usually just hundre is enough.

• hundre sider = the normal, natural way to say one hundred pages
• ett hundre sider is possible in some contexts, but it is less common and can sound more emphatic or mathematical

For ordinary speech and writing, hundre sider is the form learners should use.

A simple learner-friendly approximation is:

• Boken ≈ BOO-ken
• har ≈ har
• hundre ≈ HOON-dreh
• sider ≈ SEE-der

A few helpful notes:

• o in bok- is a long vowel
• u in hundre is not exactly like English oo, but that approximation helps at first
• r in Norwegian varies by dialect, so pronunciation can differ

If you are learning Bokmål, remember that spelling is shared by many dialects, but actual pronunciation can vary quite a lot.

Yes. Norwegian uses har in the same natural way English does here.

Just as English says:

• The book has 100 pages

Norwegian says:

• Boken har hundre sider

So this is completely idiomatic and normal.