Learn Hardcore Japanese: watasi ha memotyou wo tukue no ue ni oite okimasu.

私は = as for me / I
It tells the listener that the sentence is about me, but it doesn’t always mean I’m the grammatical “subject” in the strict sense; it’s more like “regarding me”.

In natural conversation, if it’s already clear who is speaking or who will do the action, 私 is usually omitted:

(私は) メモ帳を机の上に置いておきます。
→ Completely natural. The subject “I” is understood from context.

You’d typically keep 私は when:

You want to contrast:
私はメモ帳を置いておきますが、田中さんは持って帰ります。
Or when the subject hasn’t been established yet.

Why does メモ帳 take the particle を?

を marks the direct object of a verb — the thing the action directly affects.

メモ帳を置いておきます。
= I’ll put/leave the notepad (somewhere).

Here:

verb: 置いておきます (to put/leave something and keep it there)
direct object: メモ帳

So を simply shows that メモ帳 is what is being placed.

What does 机の上に literally mean, and how is it different from just 机に?

Literally:

机 = desk
上 = top / upper surface
机の上 = “the top of the desk,” “on the desk (as a surface)”
机の上に = “onto/on the top of the desk”

Using 机に alone would normally mean “to the desk / at the desk” rather than on its surface. For placing an object on the surface, Japanese prefers to specify the location more precisely:

机に座る – sit at the desk
机の上にメモ帳を置く – put the notepad on (top of) the desk

So 机の上に focuses on the surface; 机に does not.

Why is の used in 机の上?

の is a possessive / attributive particle, similar to “of” or an apostrophe-s in English.

机の上
= literally “the desk’s top” / “top of the desk”

This pattern is very common for locations on something:

テーブルの下 – under the table
ベッドの横 – beside the bed
家の前 – in front of the house

So の links the base object (机) and the relative position (上).

Why is the word order メモ帳を机の上に and not 机の上にメモ帳を? Are both correct?

Japanese word order is relatively flexible as long as:

The particles stay attached to their words.
The verb comes at the end.

Both are grammatical:

メモ帳を机の上に置いておきます。
机の上にメモ帳を置いておきます。

Nuance:

メモ帳を机の上に…
Slight focus on what is being moved (the notepad).
机の上にメモ帳を…
Slight focus on where it’s being placed (on the desk).

In everyday conversation, the original order with object first is very common, but neither is wrong.

What exactly does 置いておきます mean, and how is it different from just 置きます?

Breakdown:

置く – to put / place something
置いて – て-form of 置く
おく (as a helper verb) – to do something in advance, for later, or and leave it that way
置いておく – to put something somewhere and leave it there (for some purpose)
置いておきます – polite non-past form

Difference:

メモ帳を机の上に置きます。
→ I’ll put the notepad on the desk. (simple action)
メモ帳を机の上に置いておきます。
→ I’ll put/leave the notepad on the desk so it will be there when needed / as preparation / and keep it there.

〜ておく often implies:

preparation: doing something now for future convenience
leaving something in a certain state on purpose

Does 置いておきます refer to the present or the future?

Japanese polite non-past (〜ます) can cover both present and future depending on context.

Here, 置いておきます is best understood as a future-like or immediate future action:
- “I’ll (go and) put/leave it there (now/soon).”

It would not naturally mean a general habit like:

“I usually leave the notepad on the desk.”
For that, you’d likely add an adverb:
いつもメモ帳を机の上に置いておきます。 – I always leave the notepad on the desk.

Is 私 necessary here, or can I just say メモ帳を机の上に置いておきます?

You can absolutely omit 私. In fact, that’s more typical.

メモ帳を机の上に置いておきます。
is perfectly natural and usually what people would actually say.

Japanese frequently omits pronouns like “I,” “you,” “he,” “she” when they are clear from context. 私 is only needed if:

You need to emphasize “I” (contrast with someone else).
The subject is unclear and you want to make it explicit.

What is the difference between メモ帳, メモ, and ノート?

These are all related to “notes,” but they differ:

メモ帳
- A memo pad or notepad (physical small pad of paper for quick notes).
- The 帳 implies a small bound pad/book of paper.
メモ
- A note (often one note or the act of noting).
- Can be the note itself or the act of “making a memo”:
  - メモを取る – to take notes
ノート
- A notebook (usually a larger school/work notebook).
- Also means “notes” in that notebook:
  - ノートを見せてください – Please show me your notes / notebook.

In this sentence, メモ帳 emphasizes a small pad you might leave on the desk.

Why is に used after 上 (机の上に), and not で?

に often marks a target location:
- where something ends up
- where something exists (static location in many cases)
で marks the location of an action taking place.

In this sentence, the key idea is:

putting the notepad onto / into a place, and it ends up there.

So:

机の上にメモ帳を置く – put the notepad on(to) the desk (target)
If you used で, 机の上で置く would sound odd, because the action is not “taking place on top of the desk” in that sense; you are directing the notepad to that location.

For placement, に is standard:

棚に本を置く – put a book on the shelf
カバンに財布を入れる – put a wallet in the bag

Is 置いてあります possible here? What is the difference between 置いてあります and 置いておきます?

Yes, 置いてあります is possible, but it changes the meaning and time frame.

置いてあります
- from 置いてある (てある form)
- describes a state that already exists as a result of someone’s intentional action:
  - メモ帳が机の上に置いてあります。
    → The notepad is on the desk (someone has put it there on purpose and it’s there now).
置いておきます
- from 置いておく (ておく form)
- describes what you (will) do now/soon, usually as preparation:
  - メモ帳を机の上に置いておきます。
    → I’ll leave the notepad on the desk (for later / so it’s ready).

So:

置いてあります = describes an existing resulting state.
置いておきます = describes your intended action to create or maintain that state.

What is the politeness level of 置いておきます, and how does it compare to 置いておく or 置いておきました?

置いておく
- plain, dictionary form
- casual speech among friends, in writing like notes, etc.
置いておきます
- polite non-past (〜ます form)
- used in normal polite conversation, with customers, with people you’re not close to, etc.
置いておきました
- polite past (〜ました form)
- “I (have) left it there” / “I put it there (already).”

In this sentence:

置いておきます is a standard, neutral-polite way of saying:
- “I’ll leave the notepad on the desk (for you / for later).”