Learn Hardcore Japanese: konya ha yosyuu dake site, hayaku nemasu. - Tonight I will only do preparation and go to bed early.

Sets 今夜 (tonight) as the main frame/topic of the sentence.
Implies a contrast with other times, often with a nuance like “at least for tonight” or “tonight (as opposed to other nights)”.

If you said just 今夜、予習だけして、早く寝ます。 (no は), it’s still fine, but it sounds a bit more neutral and less contrastive.
今夜は subtly feels like: Tonight (at least), I’ll only do preparation and go to bed early.

Why is there no に after 今夜? Can I say 今夜に?

With many time expressions, Japanese often omits に, especially for:

general times like 毎日, 明日, 今夜, 来週, etc.

So:

今夜は予習だけして、早く寝ます。 ← most natural
今夜に予習だけして、早く寝ます。 ← usually sounds unnatural here

に is more likely when the time is:

Very specific: 7時に寝ます。 (I’ll sleep at 7.)
Used to emphasize the exact timing or contrast.

For a broad time like 今夜, you almost always say it without に in this kind of sentence.

What is 予習, and how is it different from 勉強 or 復習?

予習: “Preparing/studying before a class or lesson”
- e.g., reading the textbook chapter before tomorrow’s class
復習: “Reviewing/studying after a class or lesson”
- e.g., going over what you learned today
勉強: General “study” (not specifically before or after)

So in 今夜は予習だけして、早く寝ます。, the nuance is:

Tonight I’ll only do *preparation for future lessons, then go to bed early.* (not general study, and not review of past material)

Why is there no を after 予習? Can I say 予習をして?

予習 is a noun that combines with する:

予習する = to prepare (for a lesson)

Grammatically, you can say:

予習をして、早く寝ます。
予習して、早く寝ます。

Both are correct. In practice, を is often dropped before する-verbs, especially in speech.

In 予習だけして:

だけ (“only”) attaches directly to 予習.
You could say 予習だけをして, but that sounds a bit heavier/emphatic.
予習だけして is more natural and smooth: “just do preparation”.

What exactly does だけ mean here, and why is it placed after 予習?

だけ means “only / just / nothing but”.

予習だけして = “do only preparation (and nothing else)”

Placement:

だけ usually comes right after the thing it limits.
- 予習だけして → You are limiting 予習: you will do only preparation.
- You cannot move だけ far away; it must be next to what it modifies.

Compare:

予習だけして、早く寝ます。
→ I’ll only do preparation, then go to bed early.
予習して、早く寝るだけです。
→ All I’ll do is (prepare and go to bed early).
(Here だけ limits the whole action of preparing + going to bed.)

Also, だけ is different from しか〜ない:

予習だけして、早く寝ます。
→ neutral “I’ll only do preparation and go to bed early.”
予習しかしないで、早く寝ます。
→ stronger nuance of “I won’t do anything except preparation.”

Why is it して and not します in 予習だけして?

して is the te-form of する.

Te-form here is used to connect actions:

予習だけして、早く寝ます。
≈ “I’ll only do preparation and then go to bed early.”

In this usage, the te-form can show:

sequence: do A, then B
a light connection: A and B are part of one flow

If you used します instead:

今夜は予習だけします。早く寝ます。
→ Two separate sentences: “Tonight I’ll only do preparation. I’ll go to bed early.”

Both are fine, but the te-form version makes it feel more like one combined plan.

Can I split it into two sentences like 今夜は予習だけします。早く寝ます。? Does the meaning change?

Yes, you can say:

今夜は予習だけします。早く寝ます。

Meaning is almost the same, but nuance differs slightly:

予習だけして、早く寝ます。
→ One smooth action sequence; feels like one plan.
今夜は予習だけします。早く寝ます。
→ Two separate statements; a bit more matter‑of‑fact or step‑by‑step.

In everyday conversation, both are natural. The original te-form version just flows more as a single sentence.

What is the difference between 早い and 早く? Why is it 早く寝ます?

早い is an adjective: “early / fast”
- 早い電車 = an early train
- 早いですね。 = That’s early, isn’t it?
早く is the adverbial form of 早い: “early / quickly” (modifying verbs)
- 早く寝ます。 = I’ll sleep early.
- 早く走る。 = to run fast.

In 早く寝ます, you need the adverb form because it modifies the verb 寝ます.
早い寝ます is ungrammatical.

Does 早く here modify して or 寝ます?

In 今夜は予習だけして、早く寝ます。, 早く naturally modifies 寝ます:

The meaning is: Tonight, I’ll only do preparation and then go to bed *early.*

Clues:

早く comes right before 寝ます, separated from して by a comma.
Contextually, “do preparation early and then sleep” is less natural than “do (only) preparation and sleep early”.

So you should read it as:
[予習だけして], [早く寝ます] → first do preparation, then go to bed early.

Can I say 今晩 instead of 今夜? Is there a difference?

You can also say:

今晩は予習だけして、早く寝ます。

今夜 (こんや) and 今晩 (こんばん) both mean roughly “tonight / this evening”, and often they’re interchangeable.

Nuance (very slight, and context‑dependent):

今夜 sometimes feels a bit more “night‑time” / later at night.
今晩 is often used more broadly as “this evening/tonight”.

But in many everyday contexts (like this sentence), learners can treat them as practically the same.

Why is there no subject like 私は? How do we know who is doing the action?

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

In 今夜は予習だけして、早く寝ます。, the default assumption is:

The speaker is talking about themselves → “I (will)…”

So the full “English-style” version would be:

（私は）今夜は予習だけして、早く寝ます。

But 私は is usually dropped if it’s clear you’re talking about your own plans.
This omission is very common and natural in Japanese.