Learn Hardcore Japanese: kyou no zyugyou no pointo wo memo ni kakimasita. - I wrote the key points of today’s class in my notes.

今日の授業 → today’s class
- 今日 (today)
  - の
    - 授業 (class) → the class of today / today’s class
授業のポイント → the main points of the class
- 授業 (class)
  - の
    - ポイント (points) → the points of the class / the class’s points

Putting them together:

今日の授業のポイント → the points of today’s class

Using の twice simply chains the relationships:

(今日の授業) のポイント → the points of (the class of today)

Why is there no subject like “I” in the sentence? How do we know who wrote it?

In Japanese, the subject is often omitted when it is clear from context.

書きました is polite past tense, and in a normal conversation about your own actions, it is automatically understood as “I wrote” (or “we wrote” if you’re speaking for a group).

So:

今日の授業のポイントをメモに書きました。 → Contextually: “(I) wrote the main points of today’s class in my notes.”

If you really wanted to include the subject, you could say:

私は今日の授業のポイントをメモに書きました。 But in natural Japanese, 私 is usually dropped unless you need to contrast or clarify who did it.

Why does ポイント take を, but メモ takes に? What’s the difference between を and に here?

In this sentence:

ポイントを → を marks the direct object (what is being written).
メモに → に marks the destination / target / medium (where you are writing it).

So the structure is:

[ポイント] を [メモ] に書きました。
- ポイント = what you wrote (the content)
- メモ = where you wrote it (your notes, memo)
- 書きました = wrote

In English terms:

“I wrote the points (を) in my notes (に).”

So:

を = the thing directly acted on
に = the place/target to which the action is directed

Could you also say メモを書きました instead of メモに書きました? What’s the difference?

Yes, you can say メモを書きました, but the nuance is different.

メモを書きました。
- Focus: you wrote a memo (you created a memo/note itself).
- The memo is the thing you produced.
メモに書きました。
- Focus: you wrote something in a memo.
- The content (here, 今日の授業のポイント) is central; メモ is just the place/medium.

In your original sentence, the important thing is the points of the class, so Japanese naturally says:

今日の授業のポイントをメモに書きました。 → “I wrote the main points in my notes.”

What is the difference between メモに書きました, メモしました, and メモを取りました?

All three relate to “taking a note,” but with slightly different feelings:

メモに書きました
- Literally: “wrote (it) in a memo.”
- Explicit about writing and what was written.
- Often used when you mention what you wrote:
  - 今日の授業のポイントをメモに書きました。
メモしました
- Uses the verb メモする (“to make a memo / to take a note”).
- Short, casual, often used like “I jotted it down.”
- Doesn’t explicitly mention writing, but it’s understood.
メモを取りました
- Literally: “took a note.”
- A bit more formal or textbook-like.
- Emphasizes the act of note-taking.

In conversation, メモしました and メモに書きました are both very common; メモを取りました sounds a bit more formal or “school-like.”

Why is 書きました in the past tense? Would English “I have written …” still match that?

書きました is the polite past form of 書く (“to write”).

It indicates a completed action in the past.
Depending on context, English could translate it as:
- “I wrote …”
- “I have written …”
- “I’ve written down …”

Japanese doesn’t distinguish “simple past” versus “present perfect” like English does; 書きました just tells us “the writing is already done.” The exact English tense depends on what sounds natural in context, but the Japanese form stays 書きました.

Can I say 今日の授業のポイントを書きました without メモに? Is that still correct?

Yes, 今日の授業のポイントを書きました is grammatically correct and natural.

It means: “I wrote the main points of today’s class.”
The sentence doesn’t say where you wrote them (notebook, computer, paper, etc.).

Adding メモに simply specifies the medium:

今日の授業のポイントを書きました。 → I wrote the points. (medium not specified)
今日の授業のポイントをメモに書きました。 → I wrote the points in my notes / in a memo.

Both are fine; choice depends on whether the place/medium matters.

Can I change the word order, like メモに今日の授業のポイントを書きました? Is that more natural?

Yes, you can change the order:

今日の授業のポイントをメモに書きました。
メモに今日の授業のポイントを書きました。

Both are correct and natural. Japanese word order is flexible as long as:

The verb (書きました) stays at the end.
The particles (を, に) stay attached to the right nouns.

Often, information you want to emphasize or that is “new” can be placed earlier. For example:

If you are answering “Where did you write it?”, you might say:
- メモに今日の授業のポイントを書きました。 (emphasis on メモに)
If you are focusing on what you wrote:
- 今日の授業のポイントをメモに書きました。

What exactly does ポイント mean here, and why is it written in katakana?

ポイント comes from English “point”, and in this context it means:

key points
main points
important points

It’s written in katakana because:

It’s a loanword from English.
Katakana is the standard script for most foreign loanwords.

A more “native” Japanese word with a similar meaning is:

要点（ようてん） = main point(s), gist.

So:

今日の授業のポイント ≈ 今日の授業の要点 Both mean “the main points of today’s class,” but ポイント sounds a bit more casual and modern.

Why is there no は or が in this sentence? Could we add them?

In 今日の授業のポイントをメモに書きました, all the roles are clear from を and に, so は and が are simply not needed.

You could add は to mark a topic:

今日の授業のポイントはメモに書きました。
- Topic: 今日の授業のポイント
- Nuance: “As for the main points of today’s class, I wrote them in my notes.”
- Implication: maybe other things you didn’t write, but these you did.

Using が here is less natural unless you build a bigger sentence (e.g., 今日の授業のポイントがメモに書いてあります). With the simple 書きました, the most natural basic sentence is without は or が, just:

今日の授業のポイントをメモに書きました。

What does に mean with メモに? Is it “in,” “on,” or “to”?

The particle に has several functions; here it marks the target/destination of the action 書きました.

Conceptually, you are directing the writing toward the memo/notepad, so メモに can be translated as:

“in my notes”
“on my memo”
“into a memo”

Which English preposition you choose depends on natural English, but the Japanese idea is “the writing goes to the memo,” which is why に is used instead of で or を.

Does メモ mean “memo,” “note,” or “notebook”? How is it different from ノート?

メモ and ノート are related but not the same:

メモ
- A memo, brief note, quick jotting.
- Can be written on scrap paper, in a phone, on a sticky note, etc.
- Focus on the content (the note) rather than a full notebook object.
- メモを取る = to take notes (often quick notes).
ノート
- A notebook (physical book) or a notebook file/app.
- The object itself.
- ノートに書く = write in a notebook.

In your sentence:

メモに書きました is like “I jotted them down in my notes/a memo.” If you wanted to emphasize a notebook as the place, you could say:
今日の授業のポイントをノートに書きました。

What level of politeness is 書きました? How would it change in casual speech?

書きました is the polite past form of 書く, used in most neutral/formal situations:

With teachers
In class
With people you’re not close to
In most written Japanese

In casual speech, you would usually use:

書いた instead of 書きました

So:

Polite: 今日の授業のポイントをメモに書きました。
Casual: 今日の授業のポイントをメモに書いた。

The meaning is the same; only the politeness level changes.