Learn Hardcore Japanese: mainiti nikki wo kaku to, nihongo no hyougen ga sizen ni mi ni tukimasu. - If you write a diary every day, Japanese expressions will naturally stick with you.

A と B often means: “whenever A happens, B (naturally) happens as a result.”
It’s used for:
- natural consequences
- habitual results
- things that are generally true

So:

毎日日記を書くと、日本語の表現が自然に身につきます。
= If/When you write a diary every day, Japanese expressions will naturally stick.

Compared with other conditionals:

たら (書いたら)
- More general “if/when”.
- Can be used for one‑time future events, personal decisions, etc.
- Sounds more like “if/when you (happen to) write a diary every day, then ...”
- Still OK here, but it loses a bit of the “regular habit → natural result” feel.
なら
- “If it is the case that / as for …”
- Often used for giving advice or reacting to information:
  - 日本語を上達させたいなら、毎日日記を書くといいですよ。
    If you want to improve your Japanese, writing a diary every day is good.
ば (書けば)
- Also “if,” often a bit more formal/written.
- 毎日日記を書けば、日本語の表現が自然に身につきます。
  This is also correct and close in meaning, but と emphasizes a more automatic or expected result.

In short: と here suggests a reliable, natural consequence of a regular action.

Why is it 書く and not 書きます before と? Is this mixing polite and plain forms?

This is not mixing politeness levels incorrectly; it’s how the grammar works.

For this kind of と-conditional, the verb before と almost always appears in the plain (dictionary) present form:

✅ 書くと
❌ 書きますと (unnatural in standard Japanese)

Meanwhile, the last verb of the sentence, 身につきます, is in the polite -ます form, which sets the politeness level of the whole sentence.

So structurally it’s:

[plain form] + と、[polite form]

This is perfectly normal:

春になると、桜が咲きます。
When it becomes spring, cherry blossoms bloom.

So 書くと … つきます is grammatically and stylistically correct.

What does 身につきます literally mean, and how is it different from words like 覚えます or 上達します?

Literally:

身 (み) = body / self
身につく = “to attach to one’s body/self”

Idiomatic meaning:
身につく means “to truly acquire (a skill/knowledge) so that it becomes part of you and you can use it naturally.”

So:

日本語の表現が自然に身につきます。
= You naturally internalize Japanese expressions / they become part of you.

Compared:

覚えます: “to memorize, to learn” (often conscious effort)
- 単語をたくさん覚えました。
  I memorized many words.
上達します: “to improve, to get better (at a skill)”
- 日本語が上達しました。
  My Japanese improved.

Nuance:

身につく = you don’t just know about it; you can actually use it naturally.
It often sounds more long-term and practical, like a skill that becomes second nature.

Why is it 日本語の表現 and not just 日本語?

日本語 alone usually refers to “the Japanese language” in a broad sense (grammar, vocabulary, etc.).

表現 (ひょうげん) means “expressions; ways of saying things; phrasing.”

So:

日本語 = Japanese (the language)
日本語の表現 = Japanese expressions / ways of saying things

The sentence is talking not just about “learning Japanese in general,” but about:

acquiring natural-sounding Japanese phrases and ways to express yourself

If you said:

毎日日記を書くと、日本語が自然に身につきます。

This would be understandable and still natural, but it would sound more like:

If you write a diary every day, Japanese (overall) will naturally stick with you.

Using 日本語の表現 focuses on natural phrasing specifically, which matches the idea of writing a diary to practice expression.

Why is the particle が used with 日本語の表現, and not は or を?

In this sentence:

日本語の表現が自然に身につきます。

身につく works like:

X が身につく = “X is acquired / internalized”

So the thing that is acquired (Japanese expressions) is marked by が.

Particle roles here:

が: marks the subject / thing that experiences the change
- 日本語の表現が身につく
  Japanese expressions (subject) become internalized.
を: would usually mark a direct object of a transitive verb
- e.g. 英語を話す (speak English)
  But 身につく is intransitive; it doesn’t take を.
は: could be used to mark a topic:
- 日本語の表現は自然に身につきますよ。
  (As for Japanese expressions, they’ll naturally stick.) This is also grammatically possible but adds a contrast/topic nuance.

In neutral, explanatory statements about what gets acquired, X が身につく is the basic pattern, so が is most natural here.

What is the role of に in 自然に? Why not 自然な?

自然 (しぜん) = natural.

自然に is the adverb form: “naturally.”
自然な is the adjective form: “natural (something).”

In the sentence:

自然に身につきます。
The adverb 自然に modifies the verb 身につきます:
- [Expressions] will be acquired *naturally.*

If you used 自然な, it would need to modify a noun:

自然な日本語 = natural Japanese
自然な表現 = natural expressions

So:

自然に身につきます = They will stick *naturally (adverb → verb).*
自然な表現が身につきます = You will acquire *natural expressions (adjective → noun).*

Both are possible ideas, but in the given sentence we need an adverb, so 自然に is correct.

Why are the verbs in the present tense (書く, 身につきます) even though this is talking about the future?

Japanese often uses the present tense to describe:

general truths
habitual actions
expected future results

In this pattern:

[plain present] と、[present]

the meaning is usually:

“Whenever A happens, B happens / will happen.”

So:

毎日日記を書くと、
= If (someone) writes a diary every day / whenever (they) do that,
日本語の表現が自然に身につきます。
= Japanese expressions naturally get internalized / will be internalized.

The future meaning is understood from context:

it’s a general rule about what will happen if you make this a habit.

Japanese doesn’t need a special future tense—present is used for this kind of “general future result.”

What does 毎日 modify here—日記 or 書く? Does it matter?

In:

毎日　日記を書くと、…

毎日 can be understood as modifying the action of writing, not the diary itself:

“to write a diary *every day.”*

So the implied structure is:

(毎日) (日記を) 書く
= (every day) (a diary) write

You can rearrange the adverb slightly and keep the meaning:

日記を毎日書くと、…
毎日日記を書くと、…

Both are natural and mean:

If you write a diary every day, …

It does not mean “an everyday diary” as a noun phrase; 毎日 here is an adverb of frequency describing how often you write.

Who is the subject of the sentence? There is no 私 or あなた—how do we know who is doing the action?

Japanese often omits the subject when it’s obvious from context or when the statement is general.

The full English idea is:

If *you write a diary every day, you will naturally internalize Japanese expressions.*

In Japanese, that “you / people in general” subject is left out:

(あなたが) 毎日日記を書くと、(あなたの) 日本語の表現が自然に身につきます。

This style is common when:

giving advice
stating general rules
making impersonal statements

So the subject is generic “you / people”, understood from context, and doesn’t need to be explicitly stated.

Can I change the word order, like moving 毎日 or 日本語の表現 around, without sounding unnatural?

Yes, you have some flexibility in word order, as long as particles stay attached to the right words. Examples:

All of these are natural and mean the same thing:

毎日日記を書くと、日本語の表現が自然に身につきます。
日記を毎日書くと、日本語の表現が自然に身につきます。
日本語の表現が自然に身につきますよ。毎日日記を書くといいです。
(Split into two sentences for style.)

Key points:

Particles (を, が, に, の) must stay with the words they mark.
The main constraint is that the 最後の動詞 (last verb) carries the final meaning and politeness; here it must remain 身につきます at the end if you keep it one sentence.
Moving 毎日 or changing its position before/after 日記を is fine as long as the meaning stays clear.

So moderate reordering is OK, but don’t break the particle links or move the final verb away from the end.