Mahaba ang kabanata; marami ang pahina.

Because this sentence uses a very common Filipino pattern:

predicate + ang + topic

So:

• Mahaba ang kabanata = The chapter is long
• Marami ang pahina = The pages are many or more naturally There are many pages

English usually starts with the noun and then uses is/are, but Filipino often puts the descriptive word first.

Ang is a marker that points to the topic of the sentence, the thing being talked about.

In this sentence:

• ang kabanata = the topic being described as long
• ang pahina = the topic being described as many

It does not work exactly like English the, even though it is sometimes translated that way. Here, its main job is grammatical: it marks the noun that the predicate is about.

There is no separate word for is/are in this kind of present-tense Filipino sentence.

So instead of saying something like The chapter is long, Filipino can simply say:

Mahaba ang kabanata

That is already a complete sentence.

If you put the topic first, you can use ay:

• Ang kabanata ay mahaba
• Ang mga pahina ay marami

This sounds a bit more formal or careful than the predicate-first version.

Because mahaba and marami are being used as predicates, not as direct modifiers inside a noun phrase.

Compare these:

• mahabang kabanata = a long chapter
• mahaba ang kabanata = the chapter is long

And:

• maraming pahina = many pages
• marami ang pahina = the pages are many or there are many pages

The -ng in mahabang and maraming is a linker. It connects a modifier to the noun directly. Without that linker form, the word can stand as the predicate of the sentence.

Here, pahina is understood as pages because marami already tells you that there is more than one.

In Filipino, nouns usually do not change form for singular and plural. Context tells you which one is meant.

So:

• pahina can mean page or pages
• marami ang pahina is understood as plural because of marami

You could use mga in other contexts to mark plural, but here it is often unnecessary because the idea of plurality is already clear.

Yes. That is a correct alternative.

These two mean the same basic thing:

• Mahaba ang kabanata
• Ang kabanata ay mahaba

The difference is mainly style and word order:

• Mahaba ang kabanata is the common predicate-first pattern
• Ang kabanata ay mahaba is topic-first and can sound a bit more formal, deliberate, or written

The same idea applies to the second clause, although marami ang pahina is often the more natural shape.

The semicolon links two closely related complete clauses:

• Mahaba ang kabanata
• marami ang pahina

It shows that the two ideas belong together.

You could also write:

• Mahaba ang kabanata. Marami ang pahina.
• Mahaba ang kabanata at marami ang pahina.

The semicolon just gives the sentence a slightly more balanced, written style.