La novela tiene cuatrocientas páginas, pero mi amiga dice que es fácil de leer.

Because cuatrocientos/cuatrocientas must agree in gender with the noun it modifies.

• página is a feminine noun
• so you use the feminine form: cuatrocientas páginas

Compare:

• cuatrocientas páginas = four hundred pages
• cuatrocientos libros = four hundred books

This gender agreement happens with the hundreds in Spanish:

• doscientas páginas
• trescientas personas
• quinientos libros

Not all numbers change for gender, but the hundreds ending in -cientos / -cientas do.

In Spanish, when you talk about how many pages, rooms, years, etc. something has, you usually use tener.

So:

• La novela tiene cuatrocientas páginas = The novel has four hundred pages

This is very natural in Spanish. English sometimes also says has, so this part matches quite well.

Other examples:

• La casa tiene tres dormitorios = The house has three bedrooms
• El libro tiene muchas fotos = The book has many photos

La novela uses the definite article because we are talking about the novel, a specific one.

• la novela = the novel

But mi amiga already has a possessive:

• mi = my

In Spanish, you normally do not use the article before a possessive in this kind of structure.

So:

• mi amiga = my friend
  not la mi amiga

Compare:

• el libro = the book
• mi libro = my book

Here que means that and introduces a new clause after dice.

• mi amiga dice que es fácil de leer
• literally: my friend says that it is easy to read

In English, that is often optional:

• My friend says (that) it is easy to read

In Spanish, que is normally required here.

Other examples:

• Dice que viene mañana = She says that she is coming tomorrow
• Creo que sí = I think that yes / I think so

Because fácil here describes a quality of the novel: it is easy to read. For general characteristics like this, Spanish normally uses ser.

• es fácil de leer = it is easy to read

Using estar would usually suggest a temporary state or a different nuance, and in this sentence ser is the standard choice.

A useful way to think about it:

• ser = what something is like
• estar = how something is, often at a particular moment or condition

So here, the book’s readability is treated as a characteristic.

In Spanish, after adjectives like fácil, difícil, imposible, agradable, etc., you often use:

• adjective + de + infinitive

So:

• fácil de leer = easy to read
• difícil de entender = difficult to understand
• imposible de olvidar = impossible to forget

This is a very common structure.

So es fácil de leer is the natural Spanish way to say it is easy to read.

It refers back to la novela. The idea is:

• The novel is easy to read

Spanish does not need to say explicitly who reads it. The meaning is general: it is easy for someone to read.

This is similar to English:

• This book is easy to read
• That article is hard to understand

The person doing the action is understood from context and does not need to be stated.

Because pero means but and links two complete ideas:

• La novela tiene cuatrocientas páginas
• mi amiga dice que es fácil de leer

In Spanish, it is normal to put a comma before pero when it joins two clauses like this.

So the punctuation here is standard:

• ..., pero ...

Because fácil describes la novela, not páginas.

The subject of the second clause is understood to be la novela:

• mi amiga dice que [la novela] es fácil de leer

Since novela is singular, the adjective is singular:

• fácil

If it referred to a plural noun, it would be plural:

• Los libros son fáciles de leer = The books are easy to read

Páginas is plural because there are four hundred pages, so Spanish uses the plural noun:

• una página
• dos páginas
• cuatrocientas páginas

The accent mark in página is there because of Spanish stress rules. The word is stressed on the first syllable:

• PÁ-gi-na

Without the accent, the stress would be interpreted differently, so the written accent is necessary.

Because the sentence is stating two present facts:

• La novela tiene cuatrocientas páginas = the novel has four hundred pages
• mi amiga dice... = my friend says...

Spanish uses the present tense here just like English often does.

These are the infinitives and the forms used:

• tener → tiene
• decir → dice

Both are third-person singular forms.

Yes. Spanish word order is flexible, and you could say:

• Mi amiga dice que la novela es fácil de leer, pero tiene cuatrocientas páginas.
• La novela tiene cuatrocientas páginas, pero mi amiga dice que es fácil de leer.

Both are grammatical, but the emphasis changes.

The original sentence starts with the surprising fact:

• It has 400 pages and then contrasts it with:
• but it is easy to read

So the original version highlights the contrast very naturally.

Yes. Novela is a feminine noun, so it takes feminine words with it:

• la novela
• una novela
• esta novela
• novela larga

That is why the sentence begins with La novela.

This also matters when other words refer to it indirectly, as in:

• es fácil de leer where es refers back to la novela.