Vitabu hivyo ni vipya.

Because Swahili uses noun class agreement.
Vitabu belongs to the plural noun class that takes vi- agreement. Words connected to it must match that class:

• vitabu = books
• hivyo = those (for this noun class)
• vipya = new (for this noun class)

So the repeated vi- shows that all these words agree with vitabu.

The singular would be:

Kitabu hicho ni kipya. = That book is new.

Here the class changes from plural to singular:

• kitabu = book
• hicho = that
• kipya = new

Notice how the agreement changes from vi- to ki-.

Ni is the copula, the word that works like is / are in English.

So in this sentence, ni links:

• vitabu hivyo = those books
• vipya = new

So it functions like are in Those books are new.

In Swahili, the noun usually comes first, and words that describe or point to it usually come after it.

So:

• vitabu hivyo = those books
  not usually hivyo vitabu

This is a very common pattern in Swahili:

• noun + demonstrative
• noun + adjective

English often puts words like those before the noun, but Swahili usually puts them after.

The basic adjective root is -pya, meaning new.
It changes form to agree with the noun class.

Examples:

• kitabu kipya = a new book
• vitabu vipya = new books
• nyumba mpya = a new house / new houses

So vipya is the correct form because vitabu requires class 8 plural agreement.

These are different demonstratives for the same noun class:

• hivi = these
• hivyo = those
• vile = those over there / those yonder

So Swahili often makes a three-way distinction:

• near the speaker
• near the listener or already mentioned
• farther away

In your sentence, hivyo means those.

As a beginner, it is best to keep ni in this kind of sentence.

• Vitabu hivyo ni vipya. = Those books are new.

If you remove ni, the sentence may sound incomplete or may be understood differently depending on context. For learners, the safest pattern is:

• subject + ni
  • description

So use Vitabu hivyo ni vipya.

In Swahili, plural is often shown by the noun class prefix.

Here:

• kitabu = book
• vitabu = books

The change from ki- to vi- marks singular vs. plural for this noun class pair.

So even without an extra word like English -s, Swahili clearly shows plurality in the noun itself.

A simple pronunciation guide is:

• vitabu ≈ vee-TAH-boo
• hivyo ≈ HEE-vyo
• vipya ≈ VEE-pya

A few helpful points:

• Swahili vowels are usually very consistent:
  i as in machine, a as in father, u as in rule
• Stress usually falls on the second-to-last syllable
• In vipya, the py is said together, not separated into extra syllables

Yes. Vitabu hivyo means those books.

It is a noun phrase made of:

• vitabu = books
• hivyo = those

Then the full sentence adds more information:

• Vitabu hivyo ni vipya. = Those books are new.

So you can think of the sentence as:

• subject: vitabu hivyo
• predicate: ni vipya

Very often, yes. Agreement is a major feature of Swahili grammar.

If a noun changes class or number, the adjective usually changes too.

For example:

• kitabu kipya = new book
• vitabu vipya = new books
• mtoto mdogo = small child
• watoto wadogo = small children

So learning noun classes is important because they affect not only nouns, but also demonstratives, adjectives, and other words in the sentence.