Learn Hardcore Japanese: watasi ga rusu ni site iru aida ni, takuhaibin de nihongo no hon ga ie ni todokimasita. - While I was away, Japanese books arrived at my house by courier delivery.

私が留守にしている間に、
→ A time clause: While I was away (from home),
宅配便で日本語の本が家に届きました。
→ The main clause: a Japanese book arrived at my house by courier.

So structurally:

私が留守にしている間に = “during the time when I was away”
宅配便で = “by courier (delivery service)”
日本語の本が = “a Japanese(-language) book (subject)”
家に = “to my house”
届きました = “arrived / was delivered” (polite past)

What does 留守にしている literally mean, and why is it 留守にする rather than something like 家にいない?

留守 = “being away / not at home / absence (from home)” (a noun)
〜にする = “to make (something) into ~ / to put something into the state of ~”
留守にする = literally “to make (the house) in a state of absence,” but idiomatically “to be away from home.”

With している (progressive):

留守にしている = “(am) in the state of being away” → “am away (from home).”

So 私が留守にしている間に = “while I was away (from home).”

You could say 家にいない間に, but 留守にする is a common, natural set phrase specifically for being out / not at home.

Why is it 留守にしている (plain form) and not 留守にしています even though the main verb is 届きました (polite)?

In Japanese, subordinate clauses that modify a noun are normally in plain form, even in otherwise polite sentences.

Here, 留守にしている modifies 間:

留守にしている間 = “the period when (I) am away”

Because it’s functioning like a relative clause (“the time when I was away”), it must be plain:

✅ 私が留守にしている間に〜
❌ 私が留守にしています間に〜

The politeness of the whole sentence is carried by the final verb 届きました. Everything before that can (and usually should) be in plain form when it’s part of a clause modifying a noun (間 in this case).

Why is 私 marked with が instead of は?

In 私が留守にしている間に, 私 is the subject of the subordinate clause “(I) was away.”

私が留守にしている = “I am away”

Inside subordinate clauses that simply state who is doing the action, が normally marks the subject.

If you changed it to 私は留守にしている間に, it would sound odd here, because は would make 私 the topic of the whole sentence and clash with the structure. You want a straightforward subject in that clause, so が is natural.

There are two がs in the sentence (私が and 本が). How can a sentence have two がs?

They belong to different clauses:

私が留守にしている間に、
- 私が = subject of the subordinate clause “I was away.”
宅配便で日本語の本が家に届きました。
- 日本語の本が = subject of the main clause “a Japanese book arrived.”

Japanese often has:

[Clause with が] + 間に、[main clause with が]。

Each が is local to its own clause, so having more than one が in the full sentence is completely normal.

What is the role of に in 留守にしている? Why 留守に and not 留守を?

This is based on the pattern:

Noun + に + する = “to make (something) into N / to put it in the state of N.”

So:

留守にする = “to make (the house) be in the state of absence” → “to be away.”

留守をする also exists and can mean “to be away / to be out,” but:

留守にする is specifically “leave (somewhere) unattended / be away from (home).”
留守をする is a little broader and sometimes used like “play host, stay at home while others go out,” depending on context.

In this common “I am away from home” sense, 留守にする is very standard and natural, so you get 留守にしている.

What is the difference between 間 and 間に? Why is it 間に here?

Roughly:

〜間 = “during the whole time that ~” (the other action/state continues throughout the period)
〜間に = “at some point during the time that ~” (the other action happens once or several times within that span)

Examples:

映画を見ている間、ずっと雨が降っていました。
→ While I was watching the movie, it was raining the whole time.
映画を見ている間に、友だちから電話がありました。
→ While I was watching the movie, I got a call from a friend (at some point during it).

In your sentence:

私が留守にしている間に、…届きました。
A delivery happened at some point while you were away, not continuously, so 間に is appropriate.

What does 宅配便で mean exactly, and what is the function of で here?

宅配便 = “courier service / home delivery service” (like UPS, FedEx, Yamato, etc.)
で can mark means or method: “by / with / via.”

So:

宅配便で = “by courier,” “via a delivery service.”

It tells us how the book arrived, just like:

バスで行きます。 = “I’ll go by bus.”
メールで送りました。 = “I sent it by email.”

Why is it 日本語の本 and not 日本の本? What is the nuance?

日本語の本 = “a book in the Japanese language.”
- 日本語 = the language Japanese.
- So this is about the language the book is written in.
日本の本 = “a Japanese book” in a broader sense.
This could mean:
- a book from Japan,
- a book by a Japanese author,
- or in many contexts, also a book in Japanese.

In textbooks and clear contexts, 日本語の本 is often used when you explicitly mean “a book written in Japanese (as opposed to, say, an English book).”

What is the role of 家に here? Could we use 家へ instead?

家に届きました = “arrived at my house.”

Here, に marks the destination / target location of the delivery.

家へ is also a direction marker (“to (toward) home”), but with 届く (to be delivered, to reach), 〜に届く is the standard pattern:

✅ 家に届く (natural)
❌ 家へ届く (possible but unnatural / rarely used)

So 家に届きました is the normal, idiomatic form.

Why is 届きました used instead of 来ました or 着きました?

These verbs have different typical uses:

届く
- “to reach / to be delivered”
- Used for mail, parcels, notifications, sound, smell, etc.
- Focus: the thing successfully arrives / is delivered to a recipient.
来る
- “to come” (movement toward the speaker or a reference point)
- Used for people, objects, but doesn’t have the nuance of an official “delivery.”
着く
- “to arrive (at a place)”
- Often used for people or transportation arriving somewhere (駅に着く = arrive at the station).

For a parcel from a courier service, 届く is the natural verb:

荷物が届きました。 = “The package has arrived / been delivered.”

Could the word order be changed? For example, can we say 日本語の本が宅配便で家に届きました instead?

Yes, you can move some parts around, and it will still be grammatical:

宅配便で日本語の本が家に届きました。 (original)
日本語の本が宅配便で家に届きました。 (also natural)
日本語の本が家に宅配便で届きました。 (less common but understandable)

Japanese word order is relatively flexible, especially with adverbial phrases like 宅配便で and 家に. The main constraints:

The verb (届きました) stays at the end.
The clause 私が留守にしている間に should stay as a unit before the main clause.

Different orders change the focus slightly, but all of the above are acceptable.

Which parts of the sentence can be omitted in casual conversation?

Depending on context, Japanese often drops information that’s already clear. For example:

If it’s obvious you’re talking about yourself and your home:

（私が）留守にしている間に、（宅配便で）日本語の本が（家に）届いた。

Very casual, talking to someone who knows what you’re expecting:

留守の間に、日本語の本が届いた。
(Using 留守の間に is also common and a bit shorter.)

Possible omissions:

私が → if it’s clear it’s you.
家に → if “to my place” is obvious from context.
宅配便で → if it doesn’t matter how it came.
ます form (届いた instead of 届きました) → in casual speech.

What you cannot omit is the verb (届いた / 届きました) and the subject of the main clause (本 in some form), because those carry the core meaning “(something) arrived.”