Learn Hardcore Japanese: nikki ni sono hi no kanzyou wo mizikaku kaku to, zibun no kimoti ga sukosi wakatte kimasu. - When I briefly write my emotions of the day in my diary, I start to understand my own feelings a bit.

日記に感情を書く = write your feelings in your diary
This is the same pattern as:
- ノートに名前を書く – write your name in a notebook
- 黒板に文字を書く – write characters on the blackboard

If you said 日記を書く, that means “to write a diary / to keep a diary” (the diary itself is the thing you are writing).
In this sentence, the thing being written is 感情 (your emotions), and 日記 is the place where you write them, so 日記に is natural.

How should I understand その日の感情? What is の doing here?

その日の感情 literally means “the emotions of that day.”

The particle の here works like an of / ’s in English, linking:

その日 (that day)
感情 (emotions/feelings)

So:

その日の感情 = that day’s emotions / the emotions of that day

It shows that the emotions are specifically tied to that particular day (not just emotions in general).
You could also say その日の気持ち with a similar meaning but slightly different nuance (see next question).

What is the difference between 感情 and 気持ち in this sentence?

Both relate to feelings, but their nuance is a bit different:

感情 (かんじょう)
- More like emotions in a somewhat objective or psychological sense
- Things like joy, anger, sadness, fear – identifiable emotional states
- Here, その日の感情を書く = write down what emotions you felt that day
気持ち (きもち)
- Broader, more subjective “how you feel” internally
- Can be emotional, physical, or attitudinal (mood, comfort, motivation, etc.)
- 自分の気持ちが分かる = understand your own inner feelings / state

So the sentence is basically saying:

You record the specific emotions of that day (感情) in your diary,
and as a result, you come to understand your overall inner feelings (気持ち) a bit better.

Why is 感情 marked with を, and what does 短く modify here?

In the part その日の感情を短く書く, the structure is:

その日の感情を – direct object (what you write)
短く – adverb, “briefly / in a short way”
書く – to write

So:

感情を: the thing being written = the emotions of that day
短く書く: write (them) briefly, i.e. in a short form, not long essays

短く is the adverbial form of 短い (short), so it modifies 書く (how you write), not 感情 itself.
A literal breakdown: “If you write that day’s emotions briefly (in your diary)…”

What does と mean after 書く? How is 書くと being used?

Here と marks a conditional: “when / if (you) write …, then …”

The pattern is:

dictionary-form verb + と → a natural result or consequence

So:

日記にその日の感情を短く書くと、…
= When/If you (habitually) write that day’s emotions briefly in your diary, …

Nuance of this と:

Often used for things where the result reliably follows the condition
Feels more like a general rule / tendency, not a one-time, personal “if”
That fits the sentence, which is giving a general tip or observation:
“As a result of doing this, such-and-such tends to happen.”

Why is 書く in plain form, but 分かってきます uses the polite ます form?

In Japanese, it’s very common to:

Use plain form in subordinate/conditional clauses
Use polite form in the main clause when speaking politely

So the structure is:

(plain) 書くと – “if/when (you) write…”
(polite) 分かってきます – “you come to understand (polite)”

The politeness level is controlled by the final main verb of the sentence.
Subordinate parts like 書くと, 〜から, 〜とき, etc., are normally in plain form even in polite speech:

たくさん練習すると、上手になってきます。
時間があるとき、本を読みます。

What exactly does 自分 refer to in 自分の気持ち? Does it mean “my” or “one’s own”?

自分 (じぶん) means “oneself / oneself’s”.
Who it specifically refers to depends on context and who is being talked to/about.

In this sentence (likely giving general advice), 自分 is best understood as “your own” / “one’s own”:

自分の気持ち = your own feelings / one’s own feelings

If the speaker is clearly talking about themselves, it could be “my own feelings”, and if about another person, it could be “their own feelings.”
But with generic advice (“If you do X, you will Y”), English usually translates this 自分 as “your own”.

Why is 気持ち marked with が instead of を in 自分の気持ちが少し分かってきます?

With the verb 分かる, the thing that becomes understood is marked with が (or sometimes は), not を.

Basic pattern:

X が分かる – understand X / X becomes understandable

Examples:

日本語が分かる – (I) understand Japanese
彼の気持ちが分かる – (I) understand his feelings

So in this sentence:

自分の気持ちが分かってきます
= Your own feelings come to be understood (by you).

Japanese grammar focuses on what is getting understood (subject) rather than who is understanding (often omitted and implied as “you / I”).

What is the nuance of 分かってきます compared with just 分かります?

分かってきます is:

分かる → to understand, to become clear
分かってくる → “to come to understand,” “to gradually start to understand”
分かってきます = polite form

Nuance:

Emphasizes a gradual change over time, not instant understanding
Suggests that as you keep writing in your diary, little by little your feelings become clearer

If the sentence were 少し分かります, it would sound more like “you understand a little (at that point).”
少し分かってきます instead implies “you gradually begin to understand a bit (more and more).”

What does 少し modify here? Understanding? Feelings? The verb?

In 自分の気持ちが少し分かってきます, 少し modifies the degree of understanding, i.e. the verb phrase 分かってきます.

So it means:

“You come to understand (your feelings) a little / to some extent.”

It’s not saying that only a small part of your feelings exist; it’s saying the amount of understanding is partial or modest.

You could rephrase the nuance as:

You start to understand your feelings a bit better.

Is this sentence talking about a one-time action or a general habit?

The structure dictionary form + と + main clause (書くと、〜) usually describes a general rule, tendency, or habitual result, not a single specific event.

So the sentence is best understood as:

“When you (make it a habit to) briefly write that day’s feelings in your diary, you (tend to) come to understand your own feelings a bit.”

It’s general advice or an observation about what happens as a pattern, not “yesterday when you wrote in your diary, you understood your feelings.”

Can the word order be changed, like moving 少し or 短く somewhere else?

Some movement is possible, but it slightly changes focus or sounds less natural.

Current version:

日記にその日の感情を短く書くと、自分の気持ちが少し分かってきます。

Possible variations:

日記にその日の感情を書くと、自分の気持ちが少し分かってきます。
– Just removes 短く; still natural.
日記にその日の感情を短く書くと、少し自分の気持ちが分かってきます。
– Moves 少し forward; still understandable, but usual placement is before the verb.
日記に短くその日の感情を書くと、…
– Also possible; puts a bit more emphasis on shortly as the manner.

Japanese word order is somewhat flexible, but the original order is very natural and clear, especially for learners.