Learn Hardcore Japanese: nihongo no kyoukasyo ni kaite aru rei dake de naku, zibun de reibun wo tukuru kunren wo suru to, hyougen no haba ga hirogarimasu. - If you not only use the examples written in your Japanese textbook, but also do training in making example sentences yourself, your range of expression will expand.

に here marks the location where something is written / exists as a result:
- 本に名前を書く = to write your name in a book
- 教科書に書いてある = (something) is written in the textbook

Why not others?

を would mark the direct object of the writing action (what you write), not where it is written:
- 名前を教科書に書く = write your name in the textbook
で would usually mark the place where an action is carried out (a setting or stage), but 書いてある is not focusing on the act of writing; it’s focusing on the resulting state (“it is written there”). For that “existence/location of a result” sense, に is natural.

What exactly does 書いてある mean, and how is it different from 書いている or 書かれている?

書いてある is the てある form of 書く, and it emphasizes a resulting state caused by someone’s deliberate action.

書いてある
- “(something) is written (and left there on purpose).”
- Focus: present state + someone intentionally wrote it for a purpose
- Common for signs, instructions, examples, notes, etc.
- 教科書に例が書いてある = There are examples written in the textbook (they’ve been put there intentionally for you).
書いている
- With 書く, usually “is writing” (ongoing action), or sometimes “is written” as a state, but it doesn’t highlight intentional setup the way てある does.
- 先生が名前を書いている = The teacher is writing the name.
書かれている (passive + ている)
- More neutral “is written / has been written,” focusing on the fact itself, not so much the deliberate setup.
- ここには何も書かれていない = Nothing is written here.

In this sentence, 教科書に書いてある例 suggests “the examples that are (already) written in the textbook for you (by the authors).”

How does だけでなく work here? Is it the same as だけじゃなく or だけではなく?

だけでなく means “not only … (but also …)”.

Base pattern: A だけでなく B (も) = “not only A but (also) B.”
In the sentence:
- 日本語の教科書に書いてある例だけでなく、
  → “not only the examples written in the Japanese textbook (but also …)”

Variants:

だけでなく
- Slightly more formal/neutral.
だけではなく
- Same meaning; では can feel a bit more emphatic.
だけじゃなく
- Casual / colloquial version of だけではなく.

Functionally, they’re very similar. In written, explanatory Japanese, だけでなく or だけではなく is more common than だけじゃなく.

Where is the “but also …” part of “not only A but also B” in this sentence?

English likes an explicit “not only A, but also B” structure. In this Japanese sentence, A is marked, but B is expressed as what you do instead / in addition:

A (not only):
日本語の教科書に書いてある例だけでなく
→ not only the examples written in the Japanese textbook
B (what you should also do):
自分で例文を作る訓練をする
→ practicing making example sentences yourself

So the structure is:

Not only A, (if you) do B, C happens.

The “but also” idea is carried by the contrast between:

relying only on given examples (A) vs
actively making your own examples (B).

Why is 自分で used here, and what does で mean in 自分で例文を作る?

自分で means “by oneself / on one’s own”.

自分 = oneself
で here is the particle that often marks “means, method, or agent”.

So 自分で例文を作る literally is:

“to make example sentences by oneself”
i.e., you are the person doing it, not just copying or relying on others.

Compare:

自分の例文 = “my own example sentences” (possessive, focuses on ownership)
自分で例文を作る = “make example sentences yourself” (focuses on you being the one doing the action).

Here the emphasis is on actively creating your own sentences, so 自分で is appropriate.

What is the nuance of 訓練をする compared to 練習をする or just 練習する?

Both 訓練 and 練習 are about practice, but they feel different:

練習
- “Practice” in the ordinary sense (like practicing piano, conversation, sports drills).
- Often casual, everyday practice.
訓練
- “Training” with a stronger nuance of systematic, repeated, sometimes rigorous practice, often with a goal of building ability or discipline.
- Used for military training, fire drills, professional training, but also serious skills.

In 例文を作る訓練をする, the author is emphasizing that:

you should not just casually try it once,
but train yourself to make example sentences — as a deliberate study method.

You could say 例文を作る練習をする, which would be perfectly understandable, but 訓練 sounds a bit more serious or methodical.

What does the conditional 〜と in 訓練をすると、表現の幅が広がります mean, and how is it different from 〜たら or 〜ば?

Here 〜と marks a natural consequence or whenever X happens, Y results:

訓練をすると、表現の幅が広がります。
→ “If / when you do this training, your range of expression will expand.”

Nuance compared to others:

〜と
- Often used for results that are predictable, natural, or automatic.
- “When you press this button, the door opens.”
〜たら
- Very general “if / when.” Often used for one-time, specific situations.
- “If/when you finish your homework, let me know.”
〜ば
- Slightly more formal/abstract; often used for conditions or hypotheticals.
- “If you practice, your Japanese will improve.”

Here, the idea is that it is a natural outcome that if you keep doing this training, your expressive range will broaden. So 〜と fits well.

What does 表現の幅 literally mean, and how should I understand 表現の幅が広がります?

Breakdown:

表現 = expression (the way you express yourself: words, phrases, constructions)
幅 = width; range; breadth

So 表現の幅 is literally:

“the width of (your) expression”
→ idiomatically, “your range of expressions / your expressive range.”

表現の幅が広がります literally:

“the width of your expression widens”
→ naturally translated as “your range of expression will expand / you will be able to express yourself in more ways.”

Why is the final verb 広がります in polite form, but earlier verbs like 書いてある, 作る, する are in plain form?

In Japanese, it’s very common that:

The main clause of the sentence decides the level of politeness.
Subordinate clauses (relative clauses, conditionals, etc.) can often be in plain form even in polite sentences.

In this sentence:

Main clause (decides politeness):
- 表現の幅が広がります。 → polite form (〜ます)
Subordinate parts:
- 日本語の教科書に書いてある例 (relative clause modifying 例)
- 自分で例文を作る訓練をする (clause before と)

Those subordinate verbs (書いてある, 作る, する) don’t need to be in 〜ます form, even in a polite sentence. This mixing is natural:

たくさん本を読むと、語彙が増えます。
毎日日本語で日記を書くと、上達します。

Only the final verb that “faces the listener” must be polite when you’re speaking politely.

What is the difference between 例 and 例文 here? Why both words?

例 = example (very general; can be an example of anything)
例文 = “example sentence” (文 = sentence)

In this sentence:

書いてある例
- “the examples that are written (in the textbook).” These are probably mostly example sentences, but the word is kept general.
自分で例文を作る
- Specifically “make example sentences yourself.”

So the sentence is contrasting:

The given examples in the textbook (例)
with
The example sentences you create yourself (例文).

Using 例文 for what you create makes it crystal clear: you’re creating sentences, not just picking or listing examples in some other form.

Could the sentence be written as 自分で例文を作る訓練だけでなく…? How would that change the nuance?

If you wrote:

日本語の教科書に書いてある例だけでなく、自分で例文を作る訓練だけでなく、…

that would be odd, because you’d end up saying “not only A, not only B, …” and the structure would lose its balance.

More plausible alternatives might be:

日本語の教科書に書いてある例を覚えるだけでなく、自分で例文を作る訓練もすると…

Here, だけでなく … も is applied more symmetrically:

not only memorizing the textbook examples,
but also doing training where you make your own sentences.

In the original sentence, だけでなく only attaches to 書いてある例, and the “other side” is expressed as 自分で例文を作る訓練をする. It’s stylistically smooth and focuses the contrast on:

relying only on given examples vs
actively training yourself to make your own.

What do the commas 、 do in this sentence, especially after なく and after すると?

The commas are mainly for readability and rhythmic breaks:

…例だけでなく、自分で例文を作る訓練をすると、表現の幅が広がります。

After なく、
- Separates the “not only A” part from the following clause.
- Makes it clear where the だけでなく part ends.
After すると、
- Marks the boundary between the conditional clause (訓練をすると) and the main result clause (表現の幅が広がります).
- Helps the reader hear/see the “if … then …” structure.

You could technically omit one or both commas in casual writing, but including them is standard and makes the sentence easier to process.