datuizyo wo deta totan ni, tomodati ga onsenmanzyuu wo motte kita.

Good question. 出る is intransitive, but Japanese still often uses を with certain intransitive movement verbs to mark the place someone leaves, passes through, or moves along.

So in 脱衣所を出た, 脱衣所を means out of the changing room / leaving the changing room.

This use of を is common with verbs like:

• 家を出る = leave the house
• バスを降りる = get off the bus
• 道を歩く = walk along the road
• 橋を渡る = cross the bridge

So this を is not marking a direct object in the usual English sense. It is marking the place associated with movement.

Because とたん（に） usually attaches to the plain past form of a verb.

Pattern:

• V-た + とたん（に）

So:

• 出たとたんに = the moment someone went out / as soon as someone left

This is a fixed grammar pattern. Even though it refers to two events happening almost back-to-back, the first verb is normally in the past form.

More examples:

• 座ったとたんに、電話が鳴った。
  The moment I sat down, the phone rang.

• 窓を開けたとたんに、虫が入ってきた。
  The moment I opened the window, a bug flew in.

とたんに means something like:

• the moment ...
• just as ...
• as soon as ...

It connects two events that happen with almost no gap between them.

A very important nuance: とたん（に） often suggests that the second event happened suddenly and often unexpectedly. It is frequently used when the second event was not planned or not under the speaker’s control.

So in this sentence, the feeling is not just I left the changing room, and then my friend came. It is more like:

• The instant I left the changing room, my friend showed up with onsen manju.

That gives a stronger sense of coincidence or sudden timing.

The に in とたんに is very common, but とたん and とたんに are both used.

So you may see:

• 出たとたん、友達が来た。
• 出たとたんに、友達が来た。

Both are natural.

In many cases, the に does not change the meaning very much. It just makes the expression feel a bit more complete or set as an adverbial phrase.

For learners, the safest thing is to memorize the whole pattern as:

• V-た とたん(に)

Here, が marks 友達 as the subject of the event that suddenly happened.

Using が fits well because the sentence is presenting new event information:

• 友達が温泉まんじゅうを持って来た。
  My friend came bringing hot-spring buns.

In this kind of narrative sentence, が often sounds more natural when you are introducing who did the action as part of what happened.

If you said 友達は, it would sound more like you are making friend the topic, possibly with a contrastive feeling, such as:

• As for my friend, he came bringing onsen manju.

That is not impossible, but が is the more neutral and natural choice here.

持って来た is the past form of 持って来る.

• 持つ = hold, carry
• 来る = come

Together, 持って来る means to bring, literally to come carrying something.

So:

• 温泉まんじゅうを持って来た = brought some onsen manju / came carrying onsen manju

The direction matters:

• 持って来る = bring (toward the speaker or the current location)
• 持って行く = take (away from the speaker or current location)

Since the friend came to where the speaker was, 持って来た is the natural choice.

It can mean both, depending on context.

In many everyday sentences, 持って来る is simply the normal Japanese way to say bring:

• 本を持って来てください。
  Please bring the book.

But because it literally contains 持つ, it can also retain a slight sense of having something in hand / carrying something along.

In this sentence, the natural English translation is probably just brought, but the Japanese also gives a vivid image of the friend coming over with the onsen manju.

It is omitted, which is very normal in Japanese.

The subject of 出た is understood from context. In a sentence like this, it is often:

• the speaker
• the narrator
• or another already-known person

So the structure is basically:

• [I / we / he / she] left the changing room
• and the moment that happened, my friend came with onsen manju

Japanese frequently leaves out subjects when they are obvious from context.

They are not interchangeable.

V-たとたん(に)

• the moment...
• emphasizes immediacy
• often sudden or unexpected
• often used for things outside the speaker’s control

Example:

• ドアを開けたとたんに、猫が飛び出した。
  The moment I opened the door, a cat jumped out.

V-たら

• when / if / after
• much broader and more neutral
• can be used for planned or ordinary sequences too

Example:

• 脱衣所を出たら、友達がいた。
  When I left the changing room, my friend was there.

V-たあとで

• after doing...
• does not imply immediacy
• just says one thing happened after another

Example:

• 脱衣所を出たあとで、友達が来た。
  After I left the changing room, my friend came.

So とたんに is the best choice when you want the strong feeling of immediately after that exact moment.

Yes. That is one of the most important nuances of とたん（に）.

The second event often feels:

• sudden
• surprising
• coincidental
• not intentionally arranged by the speaker

So this sentence suggests something like:

• The instant I came out of the changing room, my friend showed up with onsen manju.

That sounds more dramatic and vivid than a plain sequence of events.

Because of that nuance, とたんに is not usually used for your own deliberate next action, such as:

• × 朝ご飯を食べたとたんに、出かけた。

That sounds odd because going out is a planned action by the same person, not a sudden event. A form like 食べたあとで or 食べてすぐ would fit better there.

It breaks down like this:

• 脱衣所を出た
  left the changing room

• とたんに
  the moment / just as

• 友達が
  my friend (subject)

• 温泉まんじゅうを
  onsen manju (object)

• 持って来た
  brought / came carrying

So the full pattern is:

• [Verb in plain past] + とたん(に), [event happened]

In other words:

• The moment [someone] left the changing room, my friend brought / came with onsen manju.