Compramos nueve libros.

Because Spanish often drops subject pronouns when the verb already shows who is doing the action.

In compramos, the ending -amos tells you the subject is we. So:

• Compramos nueve libros. = We buy / bought nine books.
• Nosotros compramos nueve libros. is also possible, but nosotros is usually only added for emphasis, contrast, or clarity.

This is very normal in Spanish, including in Spain.

It can mean both, depending on context.

Compramos is ambiguous between:

• present tense: we buy
• preterite (simple past): we bought

Examples:

• Cada mes compramos libros. = We buy books every month.
• Ayer compramos nueve libros. = Yesterday we bought nine books.

In real conversation, time words and context usually make the meaning clear.

The infinitive is comprar, meaning to buy.

Comprar is a regular -ar verb.
For nosotros (we), the form is:

• comprar → compramos

This same -amos ending appears in both:

• the present
• the preterite

That is why compramos can have two meanings.

Because numbers like nueve do not change for plural in this use.

So you say:

• nueve libros
• nueve casas
• nueve estudiantes

The noun changes to plural if needed, but nueve stays the same.

Here:

• nueve = nine
• libros = plural of libro

Because the number nueve tells us there is more than one book, so the noun must be plural.

• un libro = one book
• nueve libros = nine books

In Spanish, after numbers greater than one, the noun is normally plural.

Because Spanish usually does not use an article before a number when you are simply stating quantity.

So:

• Compramos nueve libros. = correct
• Compramos los nueve libros. = means We bought the nine books (a specific set of nine)
• Compramos unos nueve libros. = means We bought about nine books

So without an article, nueve libros just means nine books in a neutral, straightforward way.

Yes, that is grammatically correct.

But in Spanish, especially in ordinary conversation, nosotros is often unnecessary because compramos already tells you the subject is we.

You might include nosotros when:

• you want emphasis: We bought nine books, not them
• you want contrast: Nosotros compramos nueve libros, pero ellos compraron dos
• you want extra clarity

So both are correct, but Compramos nueve libros is the more neutral, natural version.

Because in Spanish, cardinal numbers like uno, dos, tres, nueve normally go before the noun they modify.

So:

• nueve libros
• tres coches
• dos mesas

This is the normal word order, just like in English:

• nine books

A simple pronunciation guide is:

kom-PRA-mos NWAY-ve LEE-bros

A few notes:

• compramos has the stress on pra
• nueve sounds roughly like NWAY-veh
• libros has the stress on li
• the r in libros is a light tap, not a strong English r

Since this is standard Spanish as used in Spain, the pronunciation is the same here as in many other varieties for these particular words.

Yes. Compramos nueve libros. is a perfectly natural and complete sentence.

It has:

• a verb: compramos
• a direct object: nueve libros

Spanish does not need an explicit subject pronoun here, so the sentence is fully grammatical and natural on its own.