Learn Hardcore Japanese: watasi ha tukue no ue ni hon wo tumimasu.

Natural full version:
私は机の上に本を積みます。 – As for me, I stack books on the desk.
Very natural everyday version:
机の上に本を積みます。 – I stack books on the desk.

In Japanese, the topic and often the subject can be omitted if it is clear from context, so just saying 机の上に本を積みます is usually more natural in a real conversation, if everyone already knows we’re talking about you.

You only need 私は when:

You want to explicitly contrast yourself with others (私は…でも彼は…)
It’s the first time you mention yourself in the conversation
You want to emphasize I specifically

Otherwise, it’s very normal to drop 私.

What’s the difference between は and が here? Why not 私が?

は marks the topic (“what we’re talking about”).
が marks the grammatical subject and often introduces new or focused information.

In your sentence:

私は → “As for me, …” (topic)
私が机の上に本を積みます。 would sound like:
“I (as opposed to someone else) am the one who stacks books on the desk.”

So 私が is used when you want to emphasize who does the action, often in contrast:

私が机の上に本を積みます。 – I’m the one who will stack the books (not you / not him).

For a neutral statement “I stack books on the desk”, 私は (or just omitting 私) is more natural.

What does 机の上 literally mean, and why do we need の?

机の上 (つくえのうえ) literally means “the top of the desk” / “the above part of the desk.”

机 = desk
上 = top / above / on
机の上 = “the top of the desk”

の here is linking two nouns: it’s like the ’s or of in English.
So:

机の上に本を積みます。 = “(I) stack books on the top of the desk.”

Without の, 机上 is a different, formal/書き言葉 compound; in normal speech you say 机の上.

Why is the particle に used after 机の上? Why not で or nothing?

Here, に marks the destination / final location where something ends up being stacked.

机の上に本を積みます。
→ The books are moved and end up on the desk.

Compare:

机の上に本を置きます。 – I put the book onto the desk.
- Focus on where it ends up → に
机の上で本を読みます。 – I read a book on top of the desk.
- The action happens at that location → で

So roughly:

Xに (for verbs like 置く, 積む, 乗せる) → “onto / into X” (result location / target)
Xで (for actions like 読む, 話す, 勉強する) → “at/in X” (place where action occurs)

You generally can’t drop the particle here;
✕ 机の上本を積みます is ungrammatical.

Could I say 机に本を積みます instead of 机の上に本を積みます?

You can say 机に本を積みます, but the nuance changes slightly:

机の上に本を積みます。
→ clear image of books stacked on the surface of the desk.
机に本を積みます。
→ more vague: “stack books on/at the desk”; it doesn’t emphasize “on the surface” as clearly.

In everyday speech, for “on the desk”, 机の上に is the most natural and explicit.

What exactly does 積みます mean? Is it just “to put”?

積みます is the polite form of 積む (つむ), which means:

to pile up, to stack, to load (like loading goods, stacking boxes, etc.)

It implies:

Putting multiple things so they form a pile/stack
Not just placing a single object once

So:

本を積みます。
→ You’re stacking books into a pile.

Related words you might see:

積む – to stack, to pile, to load
積み上げる – to stack up, to pile up (more explicit “upward piling”)
積もる – to pile up (intransitive: e.g., snow piles up)

Your sentence uses 積む, a transitive verb (you do it to something).

Why is 本 followed by を? Could we use が instead?

本を marks 本 (books) as the direct object of the verb.

本 = books
を = object marker
積みます = stack (them)

So: 本を積みます = “(I) stack books.”

If you used が:

本が積まれます。 – “Books are stacked.” (passive)
本が積んであります。 – “Books are stacked (and left that way).”

Those sentences describe the books’ state (books being stacked there), rather than what you are actively doing to them.

To say “I stack books”, you need 本を積む with を.

Can I change the word order to 本を机の上に積みます? Is that still correct?

Yes, that is perfectly correct and natural.

私は机の上に本を積みます。
私は本を机の上に積みます。

Both mean the same thing. Japanese word order is fairly flexible as long as the particles stay attached to the right words.

The default/basic order is often:

[Topic] [location/time] [object] [verb]

But you can move 本を earlier for focus or rhythm. The verb almost always comes at the end; that’s the main fixed point.

What tense is 積みます? Does it mean “I stack” or “I will stack” or “I am stacking”?

Japanese polite present (like 積みます) is broad in meaning. It can cover:

General/habitual actions
- 毎日、机の上に本を積みます。
  → I (habitually) stack books on the desk every day.
Future actions (planned / decided)
- 今から机の上に本を積みます。
  → I’ll stack books on the desk now.

It does not by itself mean “I am stacking (right now)”. For a progressive meaning you’d normally say:

今、机の上に本を積んでいます。 – I am stacking books on the desk now.

But in many contexts, 積みます can be understood as future or present from context alone.

What’s the difference between 積みます and 積む? When should I use which?

Both come from the same verb 積む (“to stack / pile up / load”).

積みます – polite, non-past form
積む – plain, non-past form

Use 積みます when:

Speaking politely to someone not very close (teacher, coworkers, strangers, customers, etc.)
In most formal writing

Use 積む when:

Talking casually with friends/family
In dictionary form, grammar explanations, or many written examples

Examples:

(polite) 私は机の上に本を積みます。
(casual) 机の上に本を積む。

Meaning is the same; it’s just a politeness difference.

If there are several books, shouldn’t we use a counter like 冊? Why is it just 本?

You use counters when you specify the number.

本を三冊積みます。 – I stack three books.
本を何冊積みますか。 – How many books do you stack?

Your original sentence 本を積みます simply says “(I) stack books” without saying how many. That’s perfectly fine and very natural when the exact quantity doesn’t matter or is obvious from context.

So:

Without a number → just 本を
With a number → 本を [number + 冊]

What’s the function of 上 here? How is it different from using 中 or 下?

上 (うえ) is a location noun meaning “top / above / on”.

Common similar nouns:

上 (うえ) – on, above, on top of
- 机の上に – on (the top of) the desk
中 (なか) – in, inside
- 机の中に – in the desk (inside a drawer, etc.)
下 (した) – under, below
- 机の下に – under the desk

They usually appear as Xの上 / Xの中 / Xの下, with の linking them to the noun.

Your sentence uses 上 to specify that the books are stacked on the surface of the desk, not inside or under it.

How would I say “Books are stacked on the desk” (describing the state, not what I do)?

To describe the state (books are already stacked there), you typically change the structure:

机の上に本が積んであります。
- Books have been stacked on the desk (and are left like that).
- Focus: someone stacked them, and they remain in that condition.
机の上に本が積まれています。
- Books are stacked on the desk. (More passive, descriptive, often written.)

Your original sentence 私が机の上に本を積みます。
focuses on your action of stacking.
The above two focus on the current state of the books on the desk.