Learn Hardcore Japanese: nikki wo nerumae ni yomikaesimasu. - I reread my diary before going to bed.

日記は寝る前に読み返します。
This makes 日記 the topic (“As for my diary, I reread it before bed”).
Grammatically both are correct; 日記を is the neutral “X is the object” form.

Why is there no subject like “I” in the sentence? How do we know it means “I reread my diary…”?

In Japanese, the subject (like “I”, “you”, “he/she”) is often omitted when it’s clear from context.

日記を寝る前に読み返します。
Literally: “Diary (object) before sleeping reread (polite).”

From context (you talking about your habits), it’s naturally understood as:

(私は) 日記を寝る前に読み返します。
→ “I reread my diary before going to bed.”

If you really want to say “I” explicitly, you can add:

私は日記を寝る前に読み返します。

What is 寝る前に doing here? Why is it 寝る前 and not 寝ます前?

寝る前に means “before sleeping / before going to bed.”

Structure:

[verb in plain/dictionary form] + 前(に) = “before doing [verb]”
- 食べる前に → before eating
- 行く前に → before going
- 寝る前に → before sleeping

Important point:

You must use the plain form of the verb (寝る, not 寝ます) before 前(に).
- ❌ 寝ます前に
- ✅ 寝る前に

So 寝る前に is a time expression: “before I go to sleep” / “before going to bed.”

What does the に after 前 do? Why not just 寝る前?

に here marks a point in time when something happens.

寝る前 on its own can mean “before sleeping” as a noun phrase (“the time before sleeping”).
寝る前に explicitly marks that time as when the action (reading) happens:
- “(I) reread my diary at the time before sleeping.”

In everyday speech, people sometimes drop the に, but 寝る前に is the more complete and clear form, and it’s what you’ll usually see in textbooks:

寝る前に日記を読み返します。
= I reread my diary before going to bed.

What’s the difference between 読みます and 読み返します?

読みます = “read” (simple reading)
読み返します = “read again”, “reread”, “read over”

The verb 読み返す is a compound verb:

読む (to read) + 返す (literally “to return”, but in compound verbs often “do again / back”)

So:

日記を寝る前に読みます。
→ “I read my diary before going to bed.” (maybe first time, or no focus on repetition)
日記を寝る前に読み返します。
→ “I reread my diary before going to bed.” (I’ve already written/read it before, and I go over it again)

How is 読み返します formed? What is the dictionary form, and how do I use it in other sentences?

The dictionary/plain form is:

読み返す (よみかえす) = to read again / reread

Conjugation pattern:

Dictionary form: 読み返す
Polite present: 読み返します
Polite past: 読み返しました
Plain present: 読み返す
Plain past: 読み返した

Examples:

昨日のメールをもう一度読み返しました。
→ I reread yesterday’s email.
この本は何回も読み返しています。
→ I have reread this book many times / I keep rereading this book.

In your sentence:

読み返します is the polite non-past form = present/future or habitual.

What tense is 読み返します? Does it mean present, future, or a habit?

Japanese -ます non-past form can cover:

Present/habitual:
- 日記を寝る前に読み返します。
  → “I (usually) reread my diary before going to bed.” (habit)
Near future (depending on context):
- 今日、寝る前に日記を読み返します。
  → “I will reread my diary before I go to bed today.”

The sentence by itself is most naturally taken as a habit/routine.

To make it clearly past:

日記を寝る前に読み返しました。
→ “I reread my diary before going to bed (on some specific occasion / yesterday, etc.).”

Can I change the word order to 寝る前に日記を読み返します? Is there any difference?

Yes, that word order is perfectly natural:

日記を寝る前に読み返します。
寝る前に日記を読み返します。

Both mean the same thing.

General idea:

Japanese word order is flexible as long as particles (を, に, は, etc.) show each word’s role.
Time expressions like 寝る前に, 毎日, 明日 often come earlier in the sentence, so many speakers slightly prefer:
- 寝る前に日記を読み返します。

But your original sentence is also correct and natural.

Could I say 日記は寝る前に読み返します? What’s the difference from 日記を寝る前に読み返します?

Yes, you can.

日記を寝る前に読み返します。
- Neutral: “I reread my diary before going to bed.”
- 日記 is just the direct object.
日記は寝る前に読み返します。
- Topic-comment structure: “As for my diary, I reread it before bed.”
- 日記 becomes the topic of the sentence.

Nuance:

Using は often contrasts with something else (explicitly or implicitly):
- “(I don’t usually reread other things, but) as for my diary, I reread it before bed.”
With を, it’s more simply “I (habitually) reread my diary before bed,” without that extra contrast feeling.

Can any parts of the sentence be omitted in real conversation?

Yes. Japanese often omits anything that’s obvious from context.

Possible omissions:

Omit the subject (most common):
- (私は) 日記を寝る前に読み返します。
  → Say just: 日記を寝る前に読み返します。
If everyone knows you’re talking about your diary, you can omit 日記:
- 寝る前に読み返します。
  → “(I) reread it before bed.”
Sometimes に after 前 is dropped in casual speech:
- 寝る前に日記を読み返します。
- 寝る前日記を読み返します。 (possible in fast speech, though less textbook-like)

For learners, it’s safest and clearest to keep the full textbook-style sentence:

日記を寝る前に読み返します。