Learn Hardcore Japanese: atarasii nikki wo katte, maiban nihongo de itigyou dake kakukoto ni simasita. - I bought a new diary and decided to write just one line in Japanese every night.

the decision 書くことにしました (decided to write) is clearly something the speaker does about their own diary and their own study habit. So the understood subject is “I”.

You could explicitly add it:

私は新しい日記を買って、…書くことにしました。

but in natural Japanese, especially about your own plans, 私は is usually dropped unless you need contrast or emphasis.

What does 日本語で mean here? Why use the particle で?

日本語で means “in Japanese” or “using Japanese.”

The particle で often marks the means, method, or tool used to do something:

バスで行く – to go by bus
箸で食べる – to eat with chopsticks
日本語で話す – to speak in Japanese

Here:

日本語で書く = “to write in Japanese”

So で is used because Japanese is the means/language you use to write.

How do you read 一行, and what does it literally mean?

一行 is read いちぎょう.

一 (いち) – one
行 (ぎょう) – line (as in a line of text)

So 一行 literally means “one line” (of writing, text, etc.).

Why is だけ placed after 一行 (一行だけ) and not before it?

In Japanese, だけ (“only / just”) usually comes after the word it limits:

一人だけ – only one person
100円だけ – only 100 yen
少しだけ – just a little

So:

一行だけ = “only one line”

Putting だけ before 一行 (like だけ一行) would sound unnatural.
You attach だけ directly to the thing you’re limiting.

Why is it 書くこと instead of just 書く or 書きます?

こと turns a verb into a noun-like phrase (a nominalization).

書く – to write
書くこと – the act of writing / writing (as a thing)

The pattern in the sentence is:

[dictionary form verb] + こと + にする

which means “decide to [verb].”

You can’t say 書くにしました; you need the こと:

一行だけ書くことにしました。
= “(I) decided to write only one line.”

So 書くこと is required by the grammar pattern 〜ことにする.

What does 書くことにしました mean as a grammar pattern?

The pattern:

[dictionary form verb] + ことにする / ことにしました

means “to decide to do [verb].”

In this sentence:

書くことにしました
→ “(I) decided to write.”

Some more examples:

毎朝走ることにしました。 – I decided to run every morning.
甘いものを食べないことにしました。 – I decided not to eat sweets.

So 書くことにしました expresses a decision the speaker has made.

Why is しました in the past tense if this is about a continuing habit?

しました is past tense because it refers to the act of deciding, which is already done.

書くことにしました。
= “I have decided to write (from now on).”

The decision is in the past, but its effect (the new habit) applies to the present and future.

Rough nuances:

書くことにします。 – I’ll decide to write (I’m deciding now / just about to).
書くことにしました。 – I decided (and that’s now my plan / rule).

So using しました is natural when you’re reporting a decision you’ve already made, even if the action will continue going forward.

What’s the difference between 書くことにしました, 書くことにしています, and 書くようにしました?

They’re related but have different nuances:

書くことにしました
- “I decided to write.”
- Focuses on the decision. The decision is done; the plan starts now.
書くことにしています
- Literally “I am making it (my rule) to write.”
- Often used to describe an ongoing personal rule or habit you currently follow.
- E.g., 毎日漢字を10個覚えることにしています。
  → “I make it a rule to learn 10 kanji every day.”
書くようにしました
- “I (have) started making an effort to write / I’ve started to write (as a new effort).”
- 〜ようにする implies trying to do something regularly, making an effort or adjustment.

In your sentence, 書くことにしました is just describing the decision itself.

Why doesn’t 毎晩 have a particle like に after it?

Time expressions like 毎晩 (every night) often appear without a particle in Japanese:

毎日学校へ行きます。 – I go to school every day.
昨日映画を見ました。 – I watched a movie yesterday.
来週友達に会います。 – I will meet a friend next week.

You normally don’t say 毎晩に書きます; 毎晩 by itself functions as a time adverb.

So:

毎晩日本語で一行だけ書くことにしました。
is natural and standard.

Can the word order be changed? For example, is 毎晩一行だけ日本語で書くことにしました also OK?

Yes, Japanese word order is fairly flexible, and several variations are natural. For example:

毎晩日本語で一行だけ書くことにしました。
毎晩一行だけ日本語で書くことにしました。
日本語で毎晩一行だけ書くことにしました。

These are all understandable and acceptable.
Small nuance differences:

毎晩日本語で一行だけ…
→ slight focus on “every night, in Japanese, one line.”
毎晩一行だけ日本語で…
→ might feel a bit more like “every night, only one line, in Japanese.”

But in everyday conversation/writing, they all basically mean the same thing. The original order is very natural and clear.

Does 日記 mean exactly “diary”? Can it also mean “journal”?

日記 (にっき) literally means “daily record” and is usually translated as “diary” or “journal,” depending on context.

It can refer to:

a private diary,
a study journal,
or any regular personal record of daily events/thoughts.

So 新しい日記を買って can naturally be understood as “I bought a new diary” or “I bought a new journal,” whichever sounds more natural in English for the situation.