asiyu ni haitta totan ni, sougeibasu ga kita.

とたんに means the moment, just as, or immediately after.

In this sentence, 入ったとたんに、送迎バスが来た means that the shuttle bus came right at the moment the speaker got into the footbath.

A key nuance of とたんに is that the second event often feels:

• very sudden
• closely timed
• somewhat unexpected

So this is not just a neutral sequence of events. It gives the feeling of As soon as I got into the footbath, of all times, the shuttle bus showed up.

With とたんに, the verb before it usually appears in the plain past form.

So:

• 入る = to enter / to get in
• 入った = entered / got in

Even though English might say the moment I get in, Japanese normally says got in + とたんに.

This pattern is very common:

• 立ち上がったとたんに = the moment I stood up
• 家を出たとたんに = the moment I left the house
• 目を閉じたとたんに = the moment I closed my eyes

So Vた + とたんに is the standard structure.

Because 入る is an intransitive verb meaning to enter or to go into, and the place entered is marked with に.

So:

• 足湯に入る = to get into a footbath
• お風呂に入る = to get into the bath
• 部屋に入る = to enter the room

English learners sometimes expect を, but 入る usually takes に for the destination/place entered.

So 足湯に入った is correct, while 足湯を入った would be wrong here.

That に is part of the expression とたんに.

You can think of:

• 〜たとたん
• 〜たとたんに

Both are used, and both mean roughly the moment that...

The に does not mark a noun like the earlier 足湯に does. It belongs to the fixed grammar pattern.

So in this sentence:

• 足湯に = into the footbath
• 入ったとたんに = the moment [someone] got in

They are two different uses of に.

足湯 is read あしゆ and means a footbath, especially a hot spring footbath where you soak only your feet.

It is made from:

• 足 = foot
• 湯 = hot water

In Japan, 足湯 often refers to a casual public footbath at:

• hot spring towns
• tourist areas
• inns
• rest stops

So 足湯に入った means got into the footbath or more naturally started soaking my feet in the footbath.

送迎バス is read そうげいバス and means a shuttle bus or pick-up/drop-off bus.

送迎 means:

• 送 = send off
• 迎 = pick up / welcome

So 送迎バス is a bus that transports people to and from a place, such as:

• a hotel
• a station
• an airport
• an event venue

In this sentence, it is the bus that the speaker was presumably waiting for or needed to catch.

Japanese often leaves out subjects when they are understood from context.

So 足湯に入った does not explicitly say:

• I got into the footbath
• we got into the footbath
• he/she got into the footbath

The subject is omitted.

In many real situations, the listener can tell from context who it is. If this sentence appears by itself, English often translates it as I got into the footbath, but that is an interpretation based on context, not a word directly stated in Japanese.

来た is the plain past form of 来る.

The sentence is talking about a completed event:

• first, someone got into the footbath
• then, at that very moment, the shuttle bus came

So 来た is used because the bus came.

If you said 来る, it would sound non-past and would not fit this completed-event narration.

Also, the sentence uses plain style:

• 入った
• 来た

If you wanted a polite version, you could say:

• 足湯に入ったとたんに、送迎バスが来ました。

They can all connect events, but the nuance is different.

〜たとたんに

• the moment X happened, Y happened
• very short time gap
• often sudden or unexpected

〜たら

• when / after / if
• broader and more neutral
• does not necessarily imply surprise or immediacy

Example:

• 足湯に入ったたら、送迎バスが来た is wrong because it should be 入ったら
• 足湯に入ったら、送迎バスが来た = When/after I got into the footbath, the shuttle bus came
  This is more neutral than とたんに.

すると

• then / and then / thereupon
• often used in storytelling
• can introduce a result or new development

So とたんに is the best choice when the speaker wants to emphasize right then and often unfortunately / unexpectedly right then.

Very often, yes.

とたんに frequently carries a sense that the second event happened:

• abruptly
• unexpectedly
• at exactly the wrong or striking moment

In this sentence, the feeling is something like:

• I had just gotten into the footbath, and then the shuttle bus came
• bad timing
• what a moment for it to arrive

That feeling does not have to be strongly negative every time, but it often sounds more dramatic than a neutral connector.

Not quite.

It is most natural when the second event is:

• sudden
• unplanned
• not under the speaker’s control

So it works well with things like:

• 雨が降り出した = it started raining
• 電話が鳴った = the phone rang
• バスが来た = the bus came

It is less natural with deliberate actions by the same person after the first event, because those are not really sudden happenings.

For example, something like:

• 家に帰ったとたんに、宿題をしました sounds unnatural, because doing homework is a planned action, not a sudden event.

So in this sentence, 送迎バスが来た fits とたんに very well.

No. Normal Japanese is written without those spaces:

足湯に入ったとたんに、送迎バスが来た。

The spaced version is just for learners, to make the parts easier to see.

The comma 、 is normal, though. It helps separate the two parts:

• 足湯に入ったとたんに
• 送迎バスが来た

Yes, that is a very good translation.

Other natural translations include:

• The moment I got into the footbath, the shuttle bus came.
• Just as I got into the footbath, the shuttle bus arrived.

If you want to reflect the nuance of とたんに, you might even say:

• The instant I got into the footbath, the shuttle bus showed up.

That version captures the suddenness a little better.