Learn Hardcore Japanese: nikki ni kyou no kibun wo kirokusuru. - I record today’s feelings in my diary.

In English, you must say “I record my mood…”
In Japanese, if it’s clear that you are talking about your own diary, you can just say:
- 日記に今日の気分を記録する。

Depending on context, this can mean:

I record my mood in my diary.
(Someone) records today’s mood in a diary.
You should record today’s mood in your diary. (if used like a general instruction)

The subject (I / you / he / she / we) is understood from the situation, not from the sentence itself.

What is the role of の in 今日の気分?

の connects 今日 (today) and 気分 (feeling/mood) in a possessive or descriptive way.

今日の気分 literally: “today’s mood”
Structure: [today] + の + [mood]

Similar patterns:

今日の天気 – today’s weather
日本の文化 – Japanese culture
私の本 – my book

So 今日の気分 is “the mood (that belongs to) today”, i.e. how you feel today.

Why do we use を after 気分?

を marks the direct object of the verb.

The verb is 記録する – to record
何を記録する？ – What do you record?
→ 今日の気分を記録する。 – (I) record today’s mood.

So:

今日の気分 = direct object (the thing being recorded)
を = object marker

Other examples:

本を読む – read a book
料理を作る – make food
音楽を聞く – listen to music

Can I change the word order, like 今日の気分を日記に記録する? Is it still correct?

Yes, you can change the order of the phrases before the verb. Both are natural:

日記に今日の気分を記録する。
今日の気分を日記に記録する。

Japanese word order rules:

The verb usually comes at the end.
The pieces before the verb (日記に / 今日の気分を) can often be rearranged without changing the core meaning.
Slight nuance:
- Putting something earlier can make it feel a bit more topical / emphasized.
- 日記に今日の気分を記録する: slightly more focus on where (in the diary).
- 今日の気分を日記に記録する: slightly more focus on what (today’s mood).

But in everyday conversation, both sound fine and basically mean the same thing.

Why use 記録する instead of just 書く here?

記録する and 書く are both possible, but they feel different:

書く – to write (very general, everyday word)
- 日記に今日の気分を書く – write today’s mood in a diary
記録する – to record / log (more formal, systematic, “for the record”)
- 日記に今日の気分を記録する – record today’s mood in a diary

Nuance of 記録する:

Sounds more objective / systematic, like logging data.
Used for logs, records, data, scores, history:
- データを記録する – record data
- 歴史を記録する – record history

So using 記録する can make the sentence feel more like:

“log your mood (as data) in a diary,”
rather than just casually “write how you feel.”

Is 日記に記録する natural Japanese, or is there a more common expression?

日記に今日の気分を記録する is grammatically correct and understandable, but for everyday speech, Japanese people more often say:

日記に今日の気分を書く。 – write today’s mood in my diary
日記をつける。 – keep a diary

More natural-sounding sentences:

日記に今日の気分を書く。
日記に、今日の気持ちを書いておく。
毎日、日記をつけている。 – I keep a diary every day.

記録する gives it a slightly technical or formal flavor, which may or may not be what you want.

Why is the verb in the plain dictionary form 記録する and not 記録します?

記録する is the plain form (dictionary form).
記録します is the polite form (ます-form).

Which one you use depends on context:

Plain: 記録する
- Used in dictionaries, notes, casual writing, inner thoughts, neutral statements.
- Often used in example sentences in textbooks.
Polite: 記録します
- Used when speaking politely, e.g. to strangers, teachers, customers.

So:

日記に今日の気分を記録する。 – neutral/plain style
日記に今日の気分を記録します。 – polite style

Both are correct; the sentence you gave is just in the plain style.

What tense is 記録する? Does it mean “record”, “will record”, or “I usually record”?

The plain dictionary form can cover several English tenses, depending on context:

Habitual / regular action:
- 日記に今日の気分を記録する。
  → I record today’s mood (in my diary) [as a routine].
Future:
- 明日から、日記に今日の気分を記録する。
  → From tomorrow, I will record today’s mood in my diary.

For past tense, you change する to した:

記録した – recorded
- 昨日、日記に今日の気分を記録した。
  → Yesterday, I recorded my mood in my diary.

So the bare 記録する itself is not locked to one tense; context decides.

Could I use で instead of に, like 日記で今日の気分を記録する?

In this sentence, に is the natural choice.
に and で have different typical roles:

に – target/destination:
- 日記に書く – write in a diary
- パソコンに保存する – save it on the computer
で – place where an action occurs, or tool/means:
- 学校で勉強する – study at school (location)
- ペンで書く – write with a pen (tool)

So:

日記に記録する = record in a diary (diary is the container/target).
日記で記録する would sound more like:
- “record using the diary as a tool/means,” which is unnatural here.

Stick with 日記に for writing/recording in a diary.

Can any particles be left out in casual speech, like dropping を?

In very casual spoken Japanese, some particles (especially は, が, を) are often dropped, if the meaning stays clear.

You might hear:

日記に今日の気分記録する。 (dropping を)

However:

In writing, especially for learners, it’s better to keep particles.
Dropping を is common in speech, but に after 日記 is usually kept here, because it’s important for showing the target.

So for correct, clear Japanese—especially as a learner—use:

日記に今日の気分を記録する。

What is the difference between 気分 and 気持ち? Could I say 今日の気持ち instead?

Both 気分 and 気持ち relate to “feeling,” but the nuance is slightly different:

気分
- often about overall mood / physical-emotional condition
- can include things like “I feel tired / sick / good / bad.”
- e.g. 気分がいい, 気分が悪い
気持ち
- more about inner emotions / feelings / thoughts
- e.g. 嬉しい気持ち, 悲しい気持ち

In this context, both are possible:

今日の気分を記録する。 – record today’s mood (overall condition)
今日の気持ちを記録する。 – record today’s feelings/emotions

The original 気分 sounds a bit like tracking mood level, maybe like a mood log.
気持ち feels slightly more emotional/psychological.

How would I turn this into a polite, natural sentence I might actually say?

A polite, natural version could be:

毎日、日記に今日の気分を記録します。
→ Every day, I record today’s mood in my diary.

Or more everyday-sounding:

毎日、日記に今日の気分を書いています。
→ Every day, I write down how I feel today in my diary.

Both are polite (〜ます form) and sound natural in normal conversation.