Learn Hardcore Japanese: watasi ha nihongo no mozi wo nooto ni kakimasu. - I write Japanese characters in my notebook.

Even though it’s written は, when used as the topic particle it’s always pronounced wa, not ha.
Inside normal words like はな (nose/flower) it’s pronounced ha; only as the topic particle it’s wa.

Why do we use 私は and not 私が here?

Both は and が can attach to 私, but they have different roles:

私は: sets me as the topic (what we’re talking about). Neutral, common, and natural here.
私が: marks me as the grammatical subject, often with a nuance like “I (as opposed to someone else) am the one who…”.

In a simple neutral statement like this, Japanese usually prefers a topic:

私は日本語の文字をノートに書きます。
= As for me, I write Japanese characters in a notebook.

If you said 私が日本語の文字をノートに書きます, it could sound like you’re emphasizing I’m the one who does it, in contrast to others.

Can I omit 私? Is 私は always needed?

You can absolutely omit 私 if it is clear from context who is doing the action. In everyday Japanese, subjects are often dropped.

So you could simply say:

日本語の文字をノートに書きます。

This would still usually be understood as I write Japanese characters in a notebook, as long as the listener knows you’re talking about yourself.
私は is often used in beginner-level examples to make the subject explicit, but in natural conversation, it would often be left out.

What does の do in 日本語の文字?

の connects two nouns and works like:

X の Y → Y of X / X’s Y / Y related to X

So:

日本語の文字
= characters of the Japanese language
= Japanese characters

Grammatically, 日本語の文字 is one noun phrase: “Japanese characters.”
日本語 (Japanese language) modifies 文字 (characters) through の.

Why say 日本語の文字 instead of just 日本語?

There’s a nuance difference:

日本語をノートに書きます。
= I write Japanese [language / sentences / words] in a notebook.
Focus is on the language content (could be sentences, words, anything in Japanese).
日本語の文字をノートに書きます。
= I write Japanese characters in a notebook.
Focus is on the characters themselves (kanji, hiragana, katakana) as writing symbols.

So if you’re practicing characters (like kanji drills), 日本語の文字 is very natural.

What is the difference between 文字, 字, and 文章?

文字（もじ）: character, letter, symbol (general term)
- Includes kanji, hiragana, katakana, even alphabet letters in some contexts.
字（じ）: also a character/letter, often more casual or used in certain compounds
- For handwriting, people might say 字がきれい (your handwriting/characters are pretty).
文章（ぶんしょう）: sentence(s), text, passage
- A longer, meaningful unit of writing, not individual characters.

In this sentence:

日本語の文字 = Japanese characters (correct for character practice).
日本語の文章 = Japanese sentences / writing (if you’re writing whole sentences).

What does the particle を do in 文字を?

を marks the direct object of the verb—the thing that the action is done to.

日本語の文字を書きます。
= (I) write Japanese characters.

So:

Verb: 書きます (write)
Direct object: 日本語の文字 (Japanese characters)
を is the bridge between them, showing what is being written.

What does ノート mean, and why is it in katakana?

ノート means notebook (or sometimes “notes”), a borrowed word from English.

It’s in katakana because:

Katakana is used for loanwords from other languages (English, etc.).
ノート comes from English “note.”

So the structure is:

ノート = notebook
ノートに書きます = write in a notebook.

Why is the particle に used with ノート? Why not で?

に has several uses, and one of them is marking a target / destination / point where something ends up.

In ノートに書きます:

ノートに: into / in the notebook (the notebook is where the writing goes)
It’s like “write in the notebook” or “write onto the notebook surface.”

Compare:

ノートに書きます。
= I write in the notebook. (The notebook is the place where the writing is recorded.)
ペンで書きます。
= I write with a pen. (で here marks the means/tool.)

Using ノートで書きます is usually unnatural if you mean “write in the notebook.”
So: ノートに (destination/place where writing appears), ペンで (tool).

Can I change the word order, like ノートに日本語の文字を書きます? Is that still correct?

Yes, Japanese word order is relatively flexible as long as the verb stays at the end and the particles are correct.

All of these are grammatically fine and mean the same thing:

私は日本語の文字をノートに書きます。
私はノートに日本語の文字を書きます。
日本語の文字をノートに書きます。
ノートに日本語の文字を書きます。

The nuance changes slightly in terms of emphasis, but they’re all natural.
The most textbook-like order is: [Topic] は [Object] を [Place] に [Verb].

What form is 書きます? How is it different from 書く?

書く is the dictionary/plain form of the verb “to write.”
書きます is the polite non-past form (the ます-form).

Functionally:

書く
- Used in casual speech, dictionaries, plain style.
書きます
- Used in polite speech (to people you’re not very close to, in class, in formal situations).

In terms of time, 書きます is “non-past,” so it can mean:

I write (habitually).
I will write.

Context decides which.

Does 書きます mean present or future? How do I say “I wrote”?

Japanese non-past (書きます / 書く) covers both:

Present: I write Japanese characters in my notebook (regularly).
Future: I will write Japanese characters in my notebook.

To say “I wrote”, you use the past polite form:

書きました = I wrote.

So:

日本語の文字をノートに書きました。
= I wrote Japanese characters in a notebook.

Why are there spaces between the Japanese words here? Are spaces normally used?

In normal Japanese writing, spaces are usually not used between words:

Standard: 私は日本語の文字をノートに書きます。

The version with spaces:

私　は　日本語　の　文字　を　ノート　に　書きます。

is typically for learners, to make it easier to see where each word/particle begins and ends.
In real books, newspapers, messages, etc., you generally won’t see spaces between Japanese words.

How do you pronounce each part of the sentence?

In romaji (one natural way):

私 – わたし – watashi
は (topic) – wa
日本語 – にほんご – nihongo
の – no
文字 – もじ – moji
を – o (often written wo, but pronounced o)
ノート – nōto (long “o”)
に – ni
書きます – かきます – kakimasu

So one smooth reading:

わたしはにほんごのもじをのーとにかきます。

Is 日本語の文字をノートに書きます the only natural way to say this, or are there common alternatives?

This sentence is natural, especially in a textbook context. Depending on nuance, you might also hear:

ノートに日本語の文字を書きます。
(same meaning, slightly different order)
ノートに日本語を書きます。
(focus more on writing Japanese language, not specifically characters)
ノートに漢字を書きます。
(specifically “I write kanji in a notebook.”)

But structurally, your original sentence is perfectly good and natural Japanese.