Learn Hardcore Japanese: nikki ni itigyou dake kansya no kotoba wo kakimasita. - I wrote just one line of words of gratitude in my diary.

日記に書く = to write in/into a diary (the diary is the place where the writing goes)
ノートに書く = to write in a notebook
黒板に書く = to write on the blackboard

Compare this with で, which usually marks the place where an action happens:

教室で日記を書きました。
I wrote my diary in the classroom. (Classroom = physical location of the action)

So:

日記に書く – the diary is the surface/medium you write on/in.
教室で書く – the classroom is the place where you do the writing.

Using 日記で書く would be strange here, because the diary is not the location of the action; it’s the destination of what you’re writing.

What exactly does 一行 (いちぎょう) mean here? Is it like “one sentence”?

一行 (いちぎょう) literally means one line (of text).

一 (いち) = one
行 (ぎょう) = line (of writing, text)

So 一行だけ = only one line.

It doesn’t necessarily mean one sentence; a sentence could be longer or shorter than one line depending on the page or font size. This expression focuses on the physical amount of writing (one line of text), not the grammatical unit (sentence).

Other examples:

三行の日記 – a three‑line diary entry
名前を一行に書いてください。 – Please write your name on one line.

How does だけ work in 一行だけ? What is being limited?

だけ means only / just and it limits what it directly follows.

Here, 一行だけ means only one line (and no more).

一行だけ感謝の言葉を書きました。
I wrote only one line of words of thanks.

If you move だけ, you change what is limited:

感謝の言葉だけを書きました。
I wrote only words of thanks (and nothing else; maybe no complaints, no diary of the day, etc.).
日記にだけ感謝の言葉を書きました。
I wrote words of thanks only in my diary (not in letters, emails, etc.).

In the original, the focus is: the amount of writing was limited to one line.

What does 感謝の言葉 literally mean? How is の working here?

感謝の言葉 (かんしゃのことば) literally means words of gratitude.

感謝 = gratitude, thanks
言葉 = word(s), phrase(s), language
の links two nouns; here it’s like of in English.

So 感謝の言葉 = words that express gratitude.

The の here is a very common way to connect two nouns:

日本の音楽 – music of Japan / Japanese music
母の手紙 – letter from my mother / my mother’s letter

So the structure is:

感謝 (gratitude)
- の
- 言葉 (words)
  → 感謝の言葉 (words of gratitude / words of thanks)

Is 感謝の言葉 a set phrase? Why not just say 感謝を書きました?

感謝の言葉 is a very natural and common expression meaning words of gratitude / a message of thanks.

Saying 感謝を書きました (literally “I wrote gratitude”) is grammatically possible but sounds unnatural and vague. 感謝 is more like an abstract feeling or concept, and Japanese usually specifies what form that feeling takes when written:

感謝の言葉を書く – write words expressing gratitude
感謝の手紙を書く – write a letter of thanks
感謝の気持ちを伝える – convey (my) feeling of gratitude

So 感謝の言葉を書きました is the idiomatic way to say you wrote something like “Thank you for …” rather than that you somehow wrote the abstract concept of gratitude itself.

Why is 言葉 marked with を and not 日記? Which noun is the direct object?

In 日記に一行だけ感謝の言葉を書きました, the direct object of the verb 書きました is 感謝の言葉.

感謝の言葉を書きました。
wrote words of gratitude.

The structure is:

日記に – where the words go (target/medium)
一行だけ – the amount (only one line)
感謝の言葉を – the thing written (direct object)
書きました – wrote

So:

日記に → target/place of writing (に)
感謝の言葉を → what is written (を, direct object)

If you said 日記を書きました, that would mean I wrote (a) diary, i.e. you wrote a diary entry itself, not something into a diary.

Why is there no subject like I in the Japanese sentence?

Japanese often omits the subject when it’s clear from context. Here, it’s natural to understand the subject as I (or we, depending on the situation).

So:

日記に一行だけ感謝の言葉を書きました。
Literally: [I] wrote only one line of words of gratitude in [my] diary.

It’s very common in Japanese to drop:

I / you / he / she / we
Possessives like my / your when obvious

You could add a subject for clarity, but it’s usually unnecessary:

私は日記に一行だけ感謝の言葉を書きました。 – perfectly correct, just more explicit.

What’s the nuance of 書きました compared to 書いた?

Both mean wrote, but:

書きました – polite past tense (です・ます style)
書いた – plain past tense (dictionary style)

Use 書きました:

In most conversations with people you’re not close to
In formal situations, customer service, writing, presentations

Use 書いた:

With close friends and family
In casual writing (notes to yourself, informal messages)
In neutral written narrative (e.g., novels often use the plain form)

So the sentence is in polite style as a whole:
日記に一行だけ感謝の言葉を書きました。

Could I say 日記に一行だけの感謝の言葉を書きました instead? Does it change the meaning?

Yes, 日記に一行だけの感謝の言葉を書きました is grammatically correct and natural. The nuance is slightly different:

一行だけ感謝の言葉を書きました。
Emphasis: I wrote only one line of words of thanks.
一行だけの感謝の言葉を書きました。
Emphasis: I wrote a one-line message of thanks (a one-line type of “words of gratitude”).

In practice, both mean almost the same thing here, and both are fine. The original is a bit more straightforward and slightly lighter sounding.

Why are there spaces between the words? I thought Japanese is normally written without spaces.

You’re right: normal Japanese writing does not use spaces between words.

The version with spaces:

日記　に　一行　だけ　感謝　の　言葉　を　書きました。

is typically used in teaching materials to help learners see where one word or particle ends and the next begins.

In natural writing, it would be:

日記に一行だけ感謝の言葉を書きました。

So the spaces are just a learning aid, not part of standard Japanese orthography.

Can に be replaced with へ here, like 日記へ書きました?

No, 日記へ書きました is not natural Japanese.

へ is used mainly with movement verbs (行く, 来る, 帰る, etc.) to show direction:
- 学校へ行きます。 – I go to school.

With 書く, when you want to express where the writing goes, you use に:

手帳にメモを書きました。 – I wrote a note in my planner.
黒板に答えを書きました。 – I wrote the answer on the blackboard.
日記に一行だけ感謝の言葉を書きました。 – I wrote only one line of words of gratitude in my diary.

So に is the correct particle here.

What’s the difference between 日記を書きました and 日記に書きました?

They express different relationships with 日記:

日記を書きました。
- 日記 is the direct object (marked by を, though it’s omitted here).
- Meaning: I wrote my diary / I wrote a diary entry.
- Focus: you produced a diary entry as a piece of writing.
日記に書きました。
- 日記に = in the diary (place/medium of writing).
- It needs an object (what you wrote) to be complete:
  - 日記に感謝の言葉を書きました。
    I wrote words of thanks in my diary.

So:

日記を書きました – The diary entry itself is what you wrote.
日記に書きました – Some other content (e.g., 感謝の言葉) was written into the diary.

Could I say this in a more casual way when talking to a friend?

Yes. A natural casual version would be:

日記に一行だけ感謝の言葉を書いた。

Changes:

書きました → 書いた (plain past)
Everything else stays the same.

You might also simplify in speech, depending on context:

日記に一行だけ書いた。 – I only wrote one line in my diary.
(What that line is may be clear from the context, so 感謝の言葉 can be dropped.)

If I want to say “I only wrote in my diary (I didn’t do anything else)”, how would I place だけ?

In the original sentence, だけ limits 一行:

一行だけ感謝の言葉を書きました。
I wrote only one line of words of thanks.

If you want only to apply to the action (writing in the diary), you could say, for example:

日記にだけ感謝の言葉を書きました。
I wrote words of thanks only in my diary (nowhere else).

To say “All I did was write in my diary”, you’d usually change the whole sentence more substantially, for example:

日記に書いただけです。 – I just wrote in my diary (that’s all I did).

So the position of だけ changes what is limited:

一行だけ – only one line
日記にだけ – only in the diary
書いただけ – only did the writing (and nothing else)