Voy a apilar estos libros en el escritorio.

This is a very common Spanish structure:

• voy = I go / I am going
• a = to
• apilar = to stack

But together, ir a + infinitive works like to be going to + verb in English.

So:

• Voy a apilar = I’m going to stack

It does not usually mean literal movement here. It is a future construction.

Examples:

• Voy a comer = I’m going to eat
• Vamos a salir = We’re going to go out

Spanish often leaves out subject pronouns because the verb already shows who is doing the action.

• voy already means I go / I’m going
• so yo is unnecessary unless you want emphasis or contrast

So both are possible:

• Voy a apilar estos libros...
• Yo voy a apilar estos libros...

The second sounds more emphatic, like:

• I’m the one who is going to stack these books
• or I am going to stack these books, not someone else

Because the future expression is:

ir + a + infinitive

So:

• voy a apilar
• vas a apilar
• va a apilar
• vamos a apilar

You cannot normally say voy apilar in standard Spanish. The a is required in this structure.

Because after voy a, Spanish uses the infinitive.

Structure:

• voy a + infinitive

So:

• voy a apilar = correct
• voy a apilo = incorrect

This is similar to English:

• I’m going to stack not
• I’m going to stacks

Yes. Apilaré means I will stack.

So these are both possible:

• Voy a apilar estos libros en el escritorio.
• Apilaré estos libros en el escritorio.

The difference is usually one of style and tone:

• voy a apilar = very common in everyday speech; often sounds more immediate or conversational
• apilaré = simple future; can sound a bit more formal, more definite, or sometimes more distant depending on context

In spoken Spanish, especially casual speech, ir a + infinitive is extremely common.

Apilar means to stack, to pile up, or to arrange in a pile.

So it is more specific than poner, which just means to put.

• Poner los libros en el escritorio = to put the books on the desk
• Apilar los libros en el escritorio = to stack the books on the desk

If the books are being placed one on top of another in an ordered pile, apilar is the better word.

Because demonstratives like este, estos, esta, estas usually go before the noun.

So:

• estos libros = these books

This is normal Spanish word order.

Compare:

• este libro = this book
• estos libros = these books
• esa mesa = that table
• aquellas casas = those houses over there

Putting estos after the noun is not the normal pattern here.

Because libros is a masculine plural noun.

Spanish demonstratives must agree with the noun in:

• gender (masculine/feminine)
• number (singular/plural)

So:

• este libro = this book
• estos libros = these books
• esta revista = this magazine
• estas revistas = these magazines

Since libros is masculine plural, estos is the correct form.

Because estos already does the job of identifying the noun.

• estos libros = these books

In English, we also do not say the these books.

So in Spanish, you normally use either:

• los libros = the books or
• estos libros = these books

but not both together in this kind of sentence.

This is a very common point for English speakers.

In Spanish, en can cover meanings that English separates into in, on, and sometimes at, depending on context.

So:

• en el escritorio here means on the desk or at the desk, depending on the situation

With surfaces like tables and desks, Spanish often uses en where English uses on.

Examples:

• Está en la mesa = It’s on the table
• Dejé el libro en el escritorio = I left the book on the desk

So this is normal Spanish usage, not a mistake.

Here it means desk.

That matters because English speakers sometimes confuse it with office.

• escritorio = desk
• oficina = office

So:

• en el escritorio = on the desk not
• in the office

In some contexts, escritorio can also refer to a writing desk or office desk specifically, but the basic meaning here is desk.

Yes, sobre el escritorio is possible, but it changes the feel slightly.

• en el escritorio = the normal everyday way to say on the desk / at the desk
• sobre el escritorio = more explicitly on top of the desk

So:

• Voy a apilar estos libros en el escritorio.
• Voy a apilar estos libros sobre el escritorio.

Both are understandable.
Usually, en el escritorio sounds more natural in everyday Spanish, while sobre can sound a bit more precise or explicit about physical position.

Yes, it is natural and correct.

A native speaker might also say:

• Voy a poner estos libros en el escritorio.
• Voy a dejar estos libros en el escritorio.
• Voy a apilar los libros en el escritorio.

The exact choice depends on what they want to emphasize:

• apilar = stack neatly/in a pile
• poner = put
• dejar = leave/place

So your sentence is perfectly fine if the idea is specifically stacking the books.

Yes. Estos normally refers to things that are close to the speaker, either physically or mentally in the conversation.

So:

• estos libros = these books (the ones here / the ones I’m referring to)

Spanish has a three-way demonstrative system:

• este / esta / estos / estas = this / these
• ese / esa / esos / esas = that / those
• aquel / aquella / aquellos / aquellas = that over there / those over there

In everyday use, the exact distance distinction is sometimes flexible, but estos generally points to something relatively near.

Yes. That is a good way to make the physical location more explicit.

• en el escritorio = on/at the desk
• sobre el escritorio = on top of the desk

If your main goal is simply to say where the books will be placed, en el escritorio is very natural.
If you want to emphasize the surface itself, sobre el escritorio works well.

Because this particular sentence uses a very straightforward Spanish structure:

• Voy a = I’m going to
• apilar = stack
• estos libros = these books
• en el escritorio = on the desk

So the order is:

verb phrase + object + place

That often matches English quite closely.

But this does not mean Spanish always follows English word order. In other sentences, Spanish can be more flexible, especially with:

• subject placement
• adjective placement
• object pronouns
• emphasis

This sentence just happens to line up neatly with English.