Learn Hardcore Japanese: konya ha nikki ni kyou no kibun wo nihongo de kakukoto ni suru. - Tonight I will write today’s feelings in my diary in Japanese.

は after a time expression like 今夜 means “as for tonight / speaking of tonight”.
If you used に (今夜に), it would sound unnatural here. Time expressions often appear without any particle or with は:
- 今夜、日記に書く。
- 今夜は日記に書く。 (adds contrast or emphasis on tonight)

Using は can imply contrast: Tonight (as opposed to other nights), I’ll write my feelings in Japanese.

Why is it 日記に and not 日記で or 日記を?

The particle に here marks the target / destination of the writing:

日記に書く = write in/into a diary
- Think: “write to the diary” or “put (the words) into the diary.”
日記で書く would mean something like “use the diary as a tool to write,” which doesn’t really make sense.
日記を could appear in other contexts (e.g. 日記を書く = write a diary as an activity), but here the focus is not on keeping a diary in general, it’s on where you write your feelings: in the diary.

So:

日記を書く = to keep/write a diary (diary is the thing you’re writing)
日記に書く = to write (something) in a diary (diary is where you put it)

Why do we have 今日の気分 when the sentence already has 今夜? Isn’t that redundant?

They refer to different things:

今夜 = tonight (the time you are writing)
今日の気分 = today’s mood / how I feel today (the content you are writing about)

So the nuance is:

Tonight, I will write (in my diary) my mood of today in Japanese.

It’s perfectly natural in Japanese (and English) to say “Tonight I’ll write about how I felt today.”

What exactly does 気分 mean here? How is it different from 気持ち?

In this sentence:

今日の気分 ≈ how I feel today / my mood today.

Nuance differences (very roughly):

気分
- Often used for overall mood, physical state, atmosphere:
  - 気分がいい = I feel good / I’m in a good mood.
  - 気分が悪い = I feel sick / I feel bad.
気持ち
- Leans more toward inner feelings / emotions / thoughts:
  - うれしい気持ち = a happy feeling.
  - 複雑な気持ち = complicated feelings.

In casual speech they overlap, but 気分 is natural for “today’s mood” in a diary context.

Why does 気分 take を? Is 今日の気分を the direct object?

Yes. 今日の気分 is the direct object of 書く:

(私は) 今夜は 日記に 今日の気分を 日本語で書く。
Subject: (私は) – I (omitted)
Object: 今日の気分を – today’s mood
Verb: 書く – to write

So 気分 is the thing that is being written, and を marks it as such.

What does 日本語で literally mean? Why use で here?

日本語で uses で to mark the means or method:

日本語で = in Japanese / using Japanese (language).
で often answers “how / by what means?”:
- 車で行く = go by car
- 英語で話す = speak in English

So:

日本語で書く = write in Japanese (using Japanese as the language).

Why is it 書くこと and not just 書く?

Here, 書くこと is a nominalized form of 書く:

書く = to write (verb)
こと = “thing” (abstract); attached to a verb, it turns it into a noun-like phrase:
- 書くこと = “the act of writing / to write (as a thing)”

This nominalized phrase then combines with にする to form the grammar pattern:

[dictionary-form verb] + ことにする = decide to do [verb]

So:

書くことにする ≈ “I will decide on the thing of writing” → I’ll decide to write.

What exactly does 書くことにする mean? How is it different from just 書く?

書くことにする is a set grammar pattern:

V-dictionary + ことにする = decide to do V / choose to do V (by my own decision).

Nuance:

書く alone: I write / I will write (simple statement of action).
書くことにする: I (have) decided that I will write – it highlights the act of deciding or choosing.

So the sentence is not just “I will write”; it’s “I’ll (make a decision that I will) write (tonight).”

What’s the difference between 書くことにする, 書くことにした, and 書こうと思う / 書くつもりだ?

All express intention, but with different nuances.

書くことにする
- Present tense.
- Sounds like you are deciding now, or presenting your plan as a decision.
- In diary / spoken: “I’ll (go with the plan to) write.”
書くことにした
- Past tense.
- Means “I decided to write.” The decision is already made (even if just now).
- Common in storytelling or reporting: I decided (that I would) write.
書こうと思う
- Volitional form + と思う.
- Nuance: I think I’ll write / I’m thinking of writing.
- More about your current intention or thought process, less formal “decision” feeling.
書くつもりだ
- V-dictionary + つもり.
- Nuance: I intend to write. Planned intention.

They overlap, but:

ことにする/した = decision wording.
～つもりだ = intention/plan.
～しようと思う = “I’m thinking I’ll…” (softer).

Why is it 書くことにする (present) when the meaning is about the future (“will decide / will write tonight”)?

Japanese often uses non-past (dictionary form) for:

Future actions,
General statements,
Decisions being made now.

So 書くことにする is naturally interpreted as:

From now (i.e., for tonight), I decide that I will write it.

If you say 書くことにした, you’re focusing on “I have decided (already)”. In this sentence, 書くことにする presents the decision as your current plan / resolution for tonight.

Is the word order flexible? Could I move parts like 今夜は or 日本語で around?

Yes, Japanese word order is relatively flexible as long as:

The verb stays at the end.
Particles stay correctly attached.

For example, you could say:

今夜は、日本語で、今日の気分を日記に書くことにする。
今日の気分を、日本語で、今夜は日記に書くことにする。

They are all understandable. The topic (は) and things you want to emphasize often come earlier.

The original order is very natural and easy to process:

Time (今夜は)
Place/target (日記に)
Object (今日の気分を)
Manner (日本語で)
Verb/decision (書くことにする)

Where is the subject “I”? How do we know it means “I will decide to write…”?

Japanese often omits the subject when it’s obvious from context.

In this sentence, the most natural subject is “I”, because:

It’s about one’s own diary.
書くことにする (decide to write) usually refers to the speaker’s decision unless context says otherwise.

So the full underlying sentence is:

（私は）今夜は日記に今日の気分を日本語で書くことにする。
- (As for me,) tonight I’ll decide to write today’s mood in my diary in Japanese.

How would this sentence look in polite form?

Just make the final verb phrase polite:

今夜は日記に今日の気分を日本語で書くことにします。

Here:

する → します (polite)
Everything else stays the same.