asita tomodati ni kodutumi wo okuru node, konya no uti ni nakami wo kakuninsimasu.

明日 is a time expression, and many common time words in Japanese can be used without a particle.

So:

• 明日 送る = send it tomorrow
• 今日 行きます = go today
• 昨日 会った = met yesterday

You can sometimes see 明日に, but in ordinary speech 明日 with no particle is much more natural here.

Here, に marks the recipient of the action.

• 友達に送る = send to a friend

With verbs like 送る (to send), に often shows the person or place something is sent to.

Compare:

• 友達に手紙を送る = send a letter to a friend
• 会社にメールを送る = send an email to the company

So in this sentence:

• 友達に = to a friend
• 小包を = the package
• 送る = send

The particle を marks the direct object—the thing being acted on.

So:

• 小包を送る = send a package

Here, the thing being sent is 小包, so it takes を.

A quick breakdown:

• 友達に = to a friend
• 小包を = the package
• 送る = send

This is a very common pattern:

• 人に 物を 送る = send something to someone

ので means something like because, since, or so. It connects a reason to the main action.

In this sentence:

• 明日 友達に 小包を 送るので
  = because I’m sending a package to a friend tomorrow / since I’m sending...

Then the result or main action follows:

• 今夜のうちに中身を確認します
  = I’ll check the contents tonight

So the whole structure is:

• Reason + ので, main action

Compared with から, ので often sounds a bit softer, more explanatory, and sometimes slightly more polite.

Yes, you could say:

• 明日友達に小包を送るから、今夜のうちに中身を確認します。

This would still make sense.

The difference is mainly nuance:

• ので = softer, more neutral, explanatory
• から = more direct, more assertive, sometimes more conversational

So:

• 送るので sounds like calmly giving a reason
• 送るから sounds a little more straightforward: because I’m sending it tomorrow

In a sentence like this, ので feels very natural.

This is very normal in Japanese.

Before ので, the verb usually appears in its plain form:

• 送るので
• 行くので
• 必要なので

The sentence can still end politely:

• 確認します

So this is not a mismatch. It is standard grammar.

You can think of it like this:

• the non-final clause uses plain form: 送るので
• the sentence-final verb carries the politeness: 確認します

This pattern is extremely common in polite Japanese.

今夜のうちに means while it is still tonight, or more naturally, before tonight is over.

The grammar ～のうちに often gives the idea of doing something within a certain time window, before the opportunity passes, or while a condition still holds.

So here:

• 今夜のうちに確認します
  = I’ll check it tonight
  with the nuance of
  I’ll make sure to do it before tonight ends

It is a bit more specific than just 今夜確認します, because it emphasizes getting it done in time.

Good question. These are not the same.

• 今夜 = tonight
• 今夜のうちに = before tonight is over / while it is still tonight

今夜に is generally not the natural choice here.

So:

• 今夜確認します = I’ll check it tonight
• 今夜のうちに確認します = I’ll check it tonight, making sure it gets done before the night ends

The のうちに adds a sense of deadline or limited time.

中身 means contents, what’s inside, or the inside contents of something.

In this sentence, it refers to what is inside the package.

Japanese nouns usually do not have separate singular/plural forms the way English does. So 中身 can be understood as:

• the contents
• what’s inside
• sometimes simply the inside

English often prefers contents here, even though the Japanese noun itself does not mark plural.

確認する means to confirm, to check, or to make sure.

So:

• 中身を確認します = check the contents / make sure what’s inside is correct

This is a very common verb in practical Japanese. It often has the nuance of verifying something carefully.

Examples:

• 予約を確認する = confirm a reservation
• 時間を確認する = check the time
• 書類を確認する = check documents

In this sentence, it suggests the speaker wants to make sure the package contents are correct before sending it.

Japanese often leaves out the subject when it is obvious from context.

In this sentence, the natural understood subject is I:

• (I) am sending a package to a friend tomorrow, so (I) will check the contents tonight.

Japanese does this all the time. If the speaker is talking about their own plans, there is usually no need to say 私は.

If you added it, it would be:

• 私は、明日友達に小包を送るので、今夜のうちに中身を確認します。

This is grammatical, but often unnecessary unless you want to emphasize the subject or clarify who is acting.

Japanese word order is more flexible than English, as long as the particles stay attached to the right words.

For example, these are also possible:

• 明日、友達に小包を送るので、今夜のうちに中身を確認します。
• 友達に明日小包を送るので、今夜のうちに中身を確認します。
• 今夜のうちに、中身を確認します。明日友達に小包を送るので。
  (more like spoken explanation, with a different flow)

But the original order is very natural.

The most important thing is that the particles show the roles:

• に = recipient
• を = object

Because of that, Japanese does not depend on word order as heavily as English does.

小包 means a parcel or package, especially something packaged and sent.

荷物 is a broader word meaning:

• luggage
• baggage
• load
• belongings
• package/shipment in some contexts

So 小包 feels more specific here: a wrapped parcel being sent to someone.

In this sentence, 小包 is a very natural choice because the speaker is sending something like a parcel to a friend.

The comma helps show a pause between the reason and the main action:

• 明日友達に小包を送るので、
• 今夜のうちに中身を確認します。

It makes the sentence easier to read, but punctuation in Japanese is somewhat flexible. The sentence could sometimes appear without a comma, especially in casual writing, but the comma is very natural here because the first part is a fairly long reason clause.