kasikiriburo wo yoyakusuru.

It is read:

かしきりぶろ を よやくする

• 貸切風呂（かしきりぶろ） = private bath / reserved bath
• を = object marker
• 予約する（よやくする） = to reserve / to book

So the whole sentence means to reserve a private bath.

貸切風呂 is a compound noun:

• 貸切（かしきり） = reserved for private use / chartered
• 風呂（ふろ） = bath

Together, 貸切風呂 means a private bath, especially one at a hotel, ryokan, or hot spring facility that you reserve for your own use.

This is not just any bathtub at home. It usually refers to a bath that can be used privately by a person, couple, or family for a set time.

Because 貸切風呂 is a single compound noun, not two separate words in a phrase.

Japanese often combines nouns like this:

• 温泉旅館 = hot spring inn
• 電車駅 would sound odd, but many noun compounds are common
• 貸切風呂 = private bath

So 貸切 is directly modifying 風呂 as part of one word. You do not say 貸切の風呂 here in the normal vocabulary item.

を marks the direct object of the verb.

In this sentence:

• 貸切風呂 = the thing being reserved
• 予約する = to reserve
• を shows that 貸切風呂 is what you are reserving

So:

• 貸切風呂を予約する = to reserve a private bath

This is very similar to English word roles in reserve a private bath, except Japanese uses the particle を to mark the object explicitly.

予約 by itself is a noun meaning reservation.

To turn it into a verb, Japanese adds する:

• 予約 = reservation
• 予約する = to make a reservation / to reserve

This is very common in Japanese. Many nouns can become verbs with する:

• 勉強する = to study
• 運動する = to exercise
• 確認する = to confirm

So 予約する is a suru-verb.

The dictionary form 予約する is the plain/basic form of the verb.

It is often used in:

• vocabulary lists
• casual speech
• written headings
• example sentences showing the basic pattern

If you wanted to say it politely in a full sentence, you would usually say:

• 貸切風呂を予約します。

That means I will reserve a private bath or I reserve a private bath, depending on context.

So 予約する is not wrong; it is just the plain form.

It can function as either, depending on context.

By itself, 貸切風呂を予約する。 feels a little like:

• a dictionary/example sentence
• a note to oneself
• a headline
• a statement in plain style

In natural conversation, Japanese often omits the subject, so it could still mean something like:

• I’ll reserve a private bath.
• We reserve the private bath.
• To reserve a private bath.

The exact meaning depends on context. Japanese often leaves out information that English usually states directly.

The subject is omitted.

Japanese very often leaves out the subject when it is obvious from context. So 貸切風呂を予約する does not explicitly say:

• I reserve...
• you reserve...
• we reserve...
• they reserve...

Any of those could be understood depending on the situation.

This is one of the biggest differences from English: Japanese often focuses on the action and object, and leaves the subject unstated.

Because Japanese is generally a subject-object-verb language, and the main verb usually comes at the end of the sentence.

So the order here is:

• 貸切風呂 = private bath
• を = object marker
• 予約する = reserve

Literally, the structure is closer to:

• private bath + object marker + reserve

This is normal Japanese word order.

Yes. That is the polite version and is very natural.

Compare:

• 貸切風呂を予約する。 = plain form
• 貸切風呂を予約します。 = polite form

The meaning is basically the same, but the tone is different.

Use 予約します when:

• speaking politely
• talking to staff
• talking to someone you do not know well
• using standard polite conversation

Not exactly.

• 貸切風呂 = a bath reserved for private use
• 個室風呂 = a bath in a private room / private-room bath
• 露天風呂（ろてんぶろ） = an open-air bath

A 貸切風呂 might be:

• indoors or outdoors
• something you reserve for a time slot
• part of a hot spring or inn

So 貸切 emphasizes private reservation, not necessarily the physical style of the bath.

Yes. In many contexts, 予約する can be translated as either reserve or book.

For example:

• ホテルを予約する = book a hotel
• レストランを予約する = reserve a restaurant table
• 貸切風呂を予約する = reserve/book a private bath

In English, book often sounds more natural in some situations, while reserve sounds more formal or specific. Japanese 予約する covers both ideas.

Yes, you can.

• 貸切風呂を予約する
• 貸切風呂の予約をする

Both are understandable and natural.

The difference is:

• 貸切風呂を予約する = directly reserve a private bath
• 貸切風呂の予約をする = make a reservation for a private bath

The second version is slightly more noun-based in structure, but the meaning is very close.

Here is a breakdown:

• 貸 = lend
• 切 = cut / complete / exclusively
• 貸切 = reserved exclusively, chartered
• 風 = wind
• 呂 = part of the kanji for bath; in 風呂, the word means bath
• 予 = beforehand
• 約 = promise / arrangement
• 予約 = reservation

You do not need to take the literal kanji meanings too strictly, but they can help you remember the words:

• 貸切 = something set aside for exclusive use
• 予約 = an arrangement made beforehand

You would say:

• 貸切風呂を予約したいです。

Breakdown:

• 予約する = to reserve
• 予約したい = want to reserve
• です = polite ending

A more casual version would be:

• 貸切風呂を予約したい。

So this sentence is a useful base pattern you can build on.