Learn Hardcore Japanese: wasurenai uti ni, kyou no zyugyou no kansou wo nikki ni kaite okimasu. - Before I forget, I’ll write my thoughts about today’s class in my diary.

忘れる前に
- More neutral and straightforward.
- Often feels like a simple time order: A happens before B.
忘れないうちに
- Emphasizes the limited window in which you still remember.
- Often feels like before it’s too late / while it’s still fresh in my mind.

In this sentence, 忘れないうちに suggests: I’d better write it now, while it’s still fresh, before my memory fades.

Why is it 忘れないうちに and not 忘れませんうちに?

Subordinate clauses (the part before うちに) usually use the plain form, not the polite form, even in polite sentences.

Plain non-past negative: 忘れない
Polite non-past negative: 忘れません

Grammar patterns like 〜うちに, 〜とき, 〜から, etc. almost always take the plain form:

忘れないうちに (correct)
✕ 忘れませんうちに (unnatural)

The sentence is still polite overall because the main verb is polite: 書いておきます.

What exactly does 〜ておきます (here: 書いておきます) mean?

Vておく has a core idea of doing something and leaving it that way; in practice it often means:

Do something in advance / as preparation
- 旅行の前に、チケットを買っておきます。
  I’ll buy the tickets (in advance) before the trip.
Do something now so you won’t have to worry later
- That’s the nuance here:
  感想を日記に書いておきます = I’ll go ahead and write my impressions in my diary (now), so it’s taken care of / so I don’t forget.

So 書いておきます ≈ I’ll write (and have it done, prepared).

Could we just say 書きます instead of 書いておきます?

You can say it, but the nuance changes:

書きます
- Simply I will write / I write.
- No special implication of preparation or getting it out of the way.
書いておきます
- I’ll write it now so it’s done / so I won’t forget / as preparation.

In the context of 忘れないうちに, 書いておきます fits very naturally because both express the idea of taking care of it before I forget.

Why is it 日記に書いておきます and not 日記で書いておきます?

The particle changes the focus:

〜に書く: write in / on / to something (target or medium)
- ノートに書く – write in a notebook
- 黒板に書く – write on the blackboard
- 日記に書く – write in a diary
〜で書く: at / in (as a place where you do the action) or with (as a tool)
- 学校で書く – write at school (place)
- ペンで書く – write with a pen (tool)

Here the diary is the place you are writing into, so 日記に is correct.

How should I understand the structure 今日の授業の感想 with two の particles?

It’s a nested AのB structure:

今日の授業 – today’s class
(今日の授業)の感想 – (the) impressions of today’s class

So:

今日の授業の感想 = impressions of today’s class.

Multiple の like this are very common:

日本の歴史の本 – a book on Japanese history
会社の上司の意見 – my boss’s opinion (from my company)

What does 感想 mean exactly? How is it different from 意見 or 気持ち?

感想
- Impressions; what you felt / thought after experiencing something.
- Often used for classes, books, movies, events:
  - 授業の感想 – impressions of the class
  - 映画の感想 – what you thought of the movie
意見
- Opinion, viewpoint, judgment about something (often more logical or argumentative).
気持ち
- Feelings, emotions, mood in a broader sense.

In 授業の感想, you’re talking about your personal impressions and thoughts after the class, not a formal debate-style opinion.

Who is the subject of this sentence? Where is I?

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

(私は) 忘れないうちに、今日の授業の感想を日記に書いておきます。

In English we must say I will write…, but in Japanese 私は is usually dropped if it’s obvious you’re talking about yourself. The polite form 〜ます also strongly suggests I in this kind of sentence.

Can うち here mean home or my place?

No. うち has several meanings:

Inside; within (as a noun), which is the one used in the grammar pattern 〜うちに.
Home; my place; our … (colloquial).

In 忘れないうちに, it’s the first one: within the time span before I forget, not at my house.

Why is it written うち in hiragana and not 内?

For this grammar pattern, it’s very common (and recommended) to write it in hiragana:

忘れないうちに (most natural)
忘れない内に (understandable but looks stiff/odd for many readers)

When 内 is used more literally as a noun meaning inside; interior, kanji is more common:

山の内側 – the inside of the mountain
会社の内情 – the inside situation of a company

For the set phrase 〜うちに, hiragana is standard in modern writing.

Does うちに always mean before something happens?

No. There are two main patterns:

V-ているうちに / V-るうちに
- While doing A, B happens (often gradually or unexpectedly).
- 日本語を勉強しているうちに、日本の文化に興味が出てきました。
  While I was studying Japanese, I gradually became interested in Japanese culture.
V-ないうちに
- Before A happens / before it ends up being A.
- 暗くならないうちに帰りましょう。
  Let’s go home before it gets dark.

The sentence uses the second pattern: 忘れないうちに (before I forget).

Why is 忘れない (non-past) used, even though forgetting will happen in the future?

Japanese often uses the non-past form (same as the present form) to talk about the future. There is no separate future tense.

忘れないうちに literally: within the time when I don’t forget
In natural English: before I forget (in the future)

So 忘れない here covers the idea of not (yet) forgetting, including the near future where forgetting might occur.

Can I change the word order, for example: 今日の授業の感想を日記に、忘れないうちに書いておきます?

Japanese word order is flexible, but there is a most natural ordering:

忘れないうちに、今日の授業の感想を日記に書いておきます。
(subordinate clause first, then main clause)

You can move 忘れないうちに later for emphasis in speech:

今日の授業の感想を日記に書いておきます、忘れないうちに。

This is conversational and a bit emphatic, like: I’ll write it in my diary — before I forget, that is. In writing or neutral speech, the original order is more standard.

Could the sentence be 日記を書いておきます instead of 日記に書いておきます?

Yes, but it slightly changes what sounds like the direct object:

日記を書いておきます
- I’ll write my diary (entry).
- The diary entry itself is the thing you are writing.
日記に書いておきます
- I’ll write (something) in my diary.
- Here, that something is 今日の授業の感想.

So in this sentence, 感想を日記に書いておきます clearly says that the impressions are what you’re writing, and the diary is where you’re writing them.