Hij leest het vijfde hoofdstuk.

Because the subject is hij (he), which takes the third-person singular form of the verb.

The infinitive is lezen (to read). In the present tense:

• ik lees = I read / I am reading
• jij leest = you read / are reading
• hij leest = he reads / is reading
• wij lezen = we read / are reading

So hij leest is the correct form.

It can mean both, depending on context.

Dutch often uses the simple present where English might use either:

• He reads the fifth chapter
• He is reading the fifth chapter

If you want to make the progressive idea extra clear in Dutch, you can also say something like:

• Hij is het vijfde hoofdstuk aan het lezen.

But the original sentence is completely normal.

Because hoofdstuk is a het-word in Dutch.

In Dutch, singular nouns usually take either:

• de
• het

You generally have to learn which article goes with each noun. For this noun:

• het hoofdstuk = the chapter

In the plural, it becomes:

• de hoofdstukken = the chapters

So even het-words usually take de in the plural.

Because vijfde is the ordinal number (fifth), while vijf is the cardinal number (five).

Compare:

• vijf hoofdstukken = five chapters
• het vijfde hoofdstuk = the fifth chapter

So if you are talking about position in a sequence, Dutch uses the ordinal form:

• eerste = first
• tweede = second
• derde = third
• vierde = fourth
• vijfde = fifth

Because it is being used like an adjective before a noun.

In het vijfde hoofdstuk, vijfde describes hoofdstuk, so it behaves like an adjective and normally takes -e.

This is very common in Dutch:

• het eerste boek = the first book
• de tweede dag = the second day
• het vijfde hoofdstuk = the fifth chapter

With ordinals before nouns, you will very often see this -e ending.

Yes. Dutch often forms compound nouns as one single word.

So:

• hoofd = head
• stuk = piece

But together hoofdstuk means chapter, not head piece in the literal English sense.

This one-word spelling is very normal in Dutch. English often uses separate words where Dutch prefers compounds.

A rough English-friendly guide would be:

• Hij ≈ a bit like hay, but not exactly; the Dutch ij is its own vowel sound
• leest ≈ like layst
• het ≈ often reduced, almost like hut or very weak ut
• vijfde ≈ roughly vayf-duh, though the Dutch ij is again not exactly English ay
• hoofdstuk ≈ roughly HOHF-stukh, with the final sound more guttural than English k

A few useful pronunciation notes:

• ij is a very common Dutch vowel spelling and does not sound exactly like any one English vowel.
• v in Dutch can sound somewhere between English v and f, depending on the speaker and region.
• The g/ch-type guttural sound in Dutch does not appear in this exact sentence much, but the end of hoofdstuk may sound sharper than an English speaker expects.

Yes, in some contexts.

• Hij leest het vijfde hoofdstuk is the more standard way to say He is reading the fifth chapter.
• Hij leest hoofdstuk vijf is also possible, especially when talking about numbered sections, chapters, exercises, pages, and so on.

There is a slight style difference:

• het vijfde hoofdstuk sounds more like an ordinal expression: the fifth chapter
• hoofdstuk vijf sounds more like a label or number: chapter five

Both are used.

Dutch main clauses follow the verb-second rule. That means the finite verb usually stays in the second position.

Original:

• Hij leest het vijfde hoofdstuk.

If you start with Vandaag (today), you get:

• Vandaag leest hij het vijfde hoofdstuk.

Notice that leest still comes second, and hij moves after the verb.

This is a very important difference from English word order.

In a subordinate clause, the conjugated verb usually moves toward the end.

Main clause:

• Hij leest het vijfde hoofdstuk.

Subordinate clause:

• Ik weet dat hij het vijfde hoofdstuk leest.
• I know that he is reading the fifth chapter.

So the word order changes:

• main clause: subject + verb + rest
• subordinate clause: subject + rest + verb

That is one of the most important Dutch sentence patterns to learn.

Not always. In everyday spoken Dutch, het is often reduced.

So while the full form is het, in natural speech it may sound weaker, almost like:

• 't

For example:

• Hij leest 't vijfde hoofdstuk.

This is very common in speech and informal writing, though learners should still know the full standard form het.

Yes. In a yes/no question, Dutch usually puts the verb first.

Statement:

• Hij leest het vijfde hoofdstuk.

Question:

• Leest hij het vijfde hoofdstuk?
• Is he reading the fifth chapter? / Does he read the fifth chapter?

This inversion is very common in Dutch questions.