rotenburo ha soto ni aru.

It is read:

ろてんぶろ は そと に ある。

• 露天風呂（ろてんぶろ） = open-air bath
• 外（そと） = outside
• ある = to exist, to be located

So the pronunciation is:

Rotenburo wa soto ni aru.

露天風呂 is a single compound noun.

• 露天（ろてん） = open air
• 風呂（ふろ / ぶろ） = bath

When combined, 風呂 becomes ぶろ because of a common sound change in compounds. So:

• 風呂 by itself: ふろ
• in 露天風呂: ぶろ

Together, 露天風呂 means open-air bath. Japanese often combines nouns like this into one word.

は marks the topic of the sentence.

So 露天風呂は means something like:

• As for the open-air bath, ...
• The open-air bath ...

The sentence is talking about the open-air bath and then saying where it is.

A very common pattern is:

A は B に ある
= A is in/at B

So here:

露天風呂は 外に ある。
= The open-air bath is outside.

Both can be possible in Japanese, but they feel different.

• 露天風呂は外にある。
  = speaking about the open-air bath as the topic

• 露天風呂が外にある。
  = puts more focus on the open-air bath itself, often as new or contrastive information

In many basic textbook-style sentences, は is used because the sentence is simply describing something about the topic.

A rough way to feel the difference:

• は = as for X
• が = X is the one that... / stronger subject focus

Because に marks the place where something exists or is located when used with ある or いる.

Pattern:

place に ある / いる

Examples:

• 部屋にある = is in the room
• 外にある = is outside
• 庭にいる = is in the garden

So in this sentence, 外に means outside as the location where the open-air bath exists.

Because ある is used for things, while いる is used for people and animals.

• ある = for objects, places, plants, abstract things
• いる = for living animate beings like people and animals

An open-air bath is a thing/place, so Japanese uses ある.

Examples:

• 温泉がある。 = There is a hot spring.
• 猫がいる。 = There is a cat.

Because ある expresses existence/location, while です is more like is/am/are for identification or description.

Compare:

• 露天風呂は外にある。
  = The open-air bath is located outside.

• 露天風呂は外です。
  This sounds less natural for talking about physical location in this case.

When saying where something is located, Japanese often prefers:

• X は place に ある
• X は place に いる

So ある is the natural choice here.

The sentence follows this pattern:

noun は place に ある

Meaning:

As for noun, it is located in/at place.

Breaking this sentence down:

• 露天風呂 = open-air bath
• は = topic marker
• 外 = outside
• に = location marker
• ある = exists / is located

So literally it is something like:

As for the open-air bath, it exists outside.

Natural English: The open-air bath is outside.

Yes. 外（そと） is basically a noun meaning outside, the outside, or outdoors.

Japanese often uses nouns where English might use adverbs or prepositional phrases.

So:

• 外にある = to be in the outside area / to be outside

Even though English says simply outside, Japanese is using 外 as a location noun plus に.

Japanese normally does not use articles like the or a/an.

So 露天風呂 could mean:

• the open-air bath
• an open-air bath

Which one is meant depends on context.

In this sentence, English often translates it as the open-air bath is outside, but Japanese itself does not explicitly mark the.

It is in the plain form because it ends in ある.

A polite version would be:

露天風呂は外にあります。

Both mean the same thing, but:

• ある = plain
• あります = polite

You would use あります in polite conversation, customer service, or formal situations.

Yes. This pattern is very flexible.

Examples:

• 露天風呂は庭にある。
  = The open-air bath is in the garden.

• 露天風呂はホテルの後ろにある。
  = The open-air bath is behind the hotel.

• 露天風呂は二階にある。
  = The open-air bath is on the second floor.

So once you know X は place に ある, you can use many different location expressions.

It means the open-air bath is located outdoors / outside.

Since 露天風呂 already means an open-air bath, the sentence may sound a little obvious in isolation, but it still works as a simple example sentence. In real life, it could make sense in a situation like:

• explaining where the bath is in a hotel or inn
• contrasting it with an indoor bath
• answering a question about location

So the sentence is grammatically natural, even if the meaning can feel a bit self-evident without context.