Het boek heeft veertig pagina's.

Because boek is a het-word in Dutch. Dutch nouns have two grammatical genders for article purposes:

• de words
• het words

So:

• het boek = the book

Unfortunately, whether a noun takes de or het often just has to be learned with the noun.

Heeft is the third-person singular present tense of hebben, which means to have.

So:

• ik heb = I have
• jij hebt = you have
• hij/zij/het heeft = he/she/it has

In Het boek heeft veertig pagina's, heeft means has.

Dutch normally uses hebben when talking about how many parts or units something contains.

So Dutch says:

• Het boek heeft veertig pagina's
  literally: The book has forty pages

That is very natural in Dutch, just as it is in English.

You could also say:

• Het boek is veertig pagina's lang

But that means something more like The book is forty pages long. The version with heeft is the most direct way to say it.

Dutch numbers usually do not change form to agree with the noun.

So veertig stays veertig no matter what noun follows:

• veertig pagina's
• veertig boeken
• veertig huizen

This is simpler than in some other languages: the number itself stays the same.

Because after numbers greater than one, Dutch normally uses the plural noun.

So:

• één pagina = one page
• twee pagina's = two pages
• veertig pagina's = forty pages

That works just like English in this case.

The apostrophe in pagina's marks the plural of a word ending in a vowel sound like -a.

Without the apostrophe, paginas could be harder to read correctly in Dutch spelling. So the correct plural is:

• pagina → pagina's

This apostrophe is not showing possession. It is only part of the plural spelling here.

No. You may also see bladzijden.

So both of these are possible:

• Het boek heeft veertig pagina's.
• Het boek heeft veertig bladzijden.

Both mean The book has forty pages.
Pagina's is very common and straightforward.

The sentence follows a very common Dutch main-clause pattern:

subject + verb + rest

Here that gives:

• Het boek = subject
• heeft = verb
• veertig pagina's = object / quantity expression

So:

• Het boek heeft veertig pagina's.

This is the normal word order for a simple statement.

Yes, that is grammatically possible, but it is less neutral.

Dutch main clauses follow the verb-second rule, which means the finite verb must come in the second position. If you move veertig pagina's to the front for emphasis, you get:

• Veertig pagina's heeft het boek.

That sounds marked or literary, with emphasis on the number of pages.
The neutral version is:

• Het boek heeft veertig pagina's.

A simple learner-friendly pronunciation is roughly:

• heeft ≈ hayft

The ee is a long vowel, and the f at the end is clearly pronounced.

So it sounds somewhat like hayft, but with a Dutch vowel quality.

A rough pronunciation guide is:

• pagina's ≈ paa-GHEE-naas

More usefully by syllable:

• pa-gi-na's

The stress is usually on gi:

• pa-GI-na's

The g in Dutch is not like the English g in go. It is a throat sound, which may take some practice.

No. In this sentence, het is the definite article, so it means the, not it.

• het boek = the book

Dutch het can also mean it in other contexts, but not here.

Not in normal standard Dutch.

You usually need the article here:

• Het boek heeft veertig pagina's.

Without het, it sounds incomplete or unnatural in this context.

The dictionary form is the singular:

• pagina = page

So:

• singular: pagina
• plural: pagina's

When learning vocabulary, it is useful to memorize both the article and the plural pattern:

• de pagina
• plural: pagina's

Because the thing doing the having is just one book.

So:

• Het boek = the book → singular subject
• veertig pagina's = forty pages → plural object

English works the same way:

• The book has forty pages

One book, many pages.