rotenburo ni haitta totan ni, yuki ga hutta.

露天風呂 is read ろてんぶろ.

It means an open-air bath or outdoor hot spring bath.

• 露天 = open air / outdoors
• 風呂 = bath

So 露天風呂に入った means got into an open-air bath.

It is used because the verb 入る often takes に when it means to enter or to get into something.

So:

• 露天風呂に入る = to get into the open-air bath
• 部屋に入る = to enter a room

A useful pattern is:

place に 入る = enter that place / get into that thing

For baths, tubs, rooms, and trains, に入る is very common.

It is 入った because とたんに usually follows the past plain form of a verb.

Pattern:

• Vた + とたんに

So:

• 入ったとたんに = the moment I got in / just as I got in

Even though 入った is grammatically past, here it does not simply mean entered as a separate finished event. In this pattern, it marks the action that happened immediately before the next one.

とたんに means the moment, just as, or as soon as.

It is used when one event happens immediately after another, often with a sense of surprise, suddenness, or bad timing.

So this sentence has the feeling of:

The moment I got into the open-air bath, it started snowing.

That gives a slightly dramatic or vivid feeling, as if the snow came at exactly that moment.

The に in とたんに is very common, but you may also see just とたん.

So both can appear:

• 入ったとたんに
• 入ったとたん

In many cases, に does not change the basic meaning very much.
とたんに is simply a standard and natural form.

In this sentence, the most natural English meaning is it started snowing.

Literally, 雪が降った is snow fell or it snowed, but because it comes after 入ったとたんに, it usually refers to the sudden beginning of the snow at that moment.

So the nuance is not just that there was snow at some point later, but that the snowfall began right then.

雪が降る is the normal way to say snow falls or it snows.

Here, 雪 is the thing doing the action of falling, so it takes が:

• 雨が降る = it rains
• 雪が降る = it snows

This is just the standard pattern with weather expressions.

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

So 入った does not explicitly say I entered, he entered, or we entered. The listener figures it out from the situation.

In many textbook translations, this sentence would be understood as:

• The moment I got into the open-air bath, it started snowing.

But depending on context, it could also mean:

• The moment he got into the open-air bath, it started snowing.

Japanese does this very often.

Yes. とたんに is much more specific and dramatic than とき.

Compare:

• 露天風呂に入ったとき、雪が降った。
  = When I got into the open-air bath, it snowed.

• 露天風呂に入ったとたんに、雪が降った。
  = The very moment I got into the open-air bath, it started snowing.

とき just gives a time frame.
とたんに emphasizes immediacy and often a surprising coincidence.

You could, but the nuance would change.

• 入ったら、雪が降った。
  This can mean when/after I got in, it snowed, but it does not strongly emphasize that exact instant.

• 入ると、雪が降った。
  This sounds less natural here, because と is often used for regular results, natural consequences, or things that happen whenever X happens. It can also sound more objective or habitual.

• 入ったとたんに、雪が降った。
  Best if you want the very moment feeling.

So とたんに is especially good here because the sentence describes a sudden event happening right away.

Yes. In Japanese, 入る is the standard verb for getting into a bath.

So:

• お風呂に入る = take a bath / get in the bath
• 温泉に入る = get into a hot spring
• 露天風呂に入る = get into an open-air bath

This is one of those expressions that may feel different from English, where we often say take a bath. In Japanese, the idea is more literally enter the bath.

Usually no. とたんに is mainly used when the second event happens suddenly and not as a deliberate action by the same person.

It works well for things like:

• lights going out
• rain starting
• someone appearing
• a phone ringing

So this sentence is natural because snow started falling is an unexpected event.

A less natural use would be something like:

• 部屋に入ったとたんに、宿題を始めた。

That sounds odd, because starting homework is a deliberate action, not a sudden outside event.

A polite version could be:

露天風呂に入ったとたんに、雪が降りました。

Or, if you want the whole sentence to sound more polite and narrative:

露天風呂に入ったとたんに、雪が降ってきました。

The second version can sound especially natural if you want to express it began to snow.

But the original plain form is completely normal in casual speech, storytelling, or written narration.