El libro es muy gordo.

In Spanish, a singular countable noun usually needs an article or some other determiner.

So el libro means the book. Saying just libro by itself would usually sound incomplete in a normal sentence.

English often allows zero article in places where Spanish does not, so this is a very common difference for learners.

Libro is a masculine noun, so it takes the masculine article el.

A very common pattern is:

• nouns ending in -o are often masculine
• nouns ending in -a are often feminine

So:

• el libro
• el coche
• la casa
• la mesa

This is a pattern, not a perfect rule, but it works here.

Spanish has two verbs for to be: ser and estar.

Here, es is from ser, and it is used because the sentence describes a characteristic of the book.

• es = from ser
• está = from estar

With a description like gordo in this sentence, ser is the normal choice.

So:

• El libro es muy gordo = the book is very thick/fat

Using está here would sound wrong in standard Spanish.

Es is the he / she / it is form of ser in the present tense.

Full present tense of ser:

• yo soy
• tú eres
• él / ella / usted es
• nosotros somos
• vosotros sois
• ellos / ellas / ustedes son

Because el libro is a singular noun, Spanish uses the same form as it is: es.

Because muy is an adverb meaning very, and in Spanish adverbs like this normally go before the adjective they modify.

So:

• muy gordo = very thick / very fat

This works just like:

• muy grande = very big
• muy interesante = very interesting
• muy caro = very expensive

No. Muy never changes.

It stays muy with masculine, feminine, singular, and plural nouns:

• el libro es muy gordo
• la revista es muy gorda
• los libros son muy gordos
• las revistas son muy gordas

The adjective changes, but muy does not.

Because the adjective must agree with the noun it describes.

Libro is masculine singular, so the adjective must also be masculine singular:

• libro gordo

If the noun were feminine singular, it would be:

• revista gorda

If it were masculine plural:

• libros gordos

If it were feminine plural:

• revistas gordas

This is called adjective agreement.

It can be, especially in a figurative or informal way. Literally, gordo often means fat, but Spanish can also use it for things that are thick or bulky.

So for a book, gordo can mean something like:

• thick
• fat
• chunky

A more literal word for thick is often grueso:

• El libro es muy grueso

So gordo is understandable and natural in many contexts, but grueso may sound more directly like thick to learners.

Not in this sentence as it stands, because the adjective is part of the structure noun + ser + adjective:

• El libro es muy gordo

You are not placing the adjective directly before the noun; you are using it after the verb es.

If you wanted the adjective directly with the noun, Spanish usually puts descriptive adjectives after the noun:

• el libro gordo

Putting gordo before libro would sound unusual here.

Yes. If you remove muy, the sentence becomes:

• El libro es gordo

That means The book is thick/fat.

Adding muy just strengthens the description:

• El libro es muy gordo = The book is very thick

Both the verb and the adjective change to match the plural noun.

Singular:

• El libro es muy gordo

Plural:

• Los libros son muy gordos

Changes:

• el → los
• es → son
• gordo → gordos

This is another example of agreement in Spanish.

A simple pronunciation guide is:

el LEE-bro es mwee GOR-do

A more Spain-focused approximation:

• el = like el
• libro = LEE-bro
• es = ess
• muy = roughly mwee
• gordo = GOR-do

In much of Spain, the d in gordo is softer than an English d, especially between vowels.