doa ha sono syunkan ni aita.

In normal Japanese writing, there are usually no spaces between words.

So this sentence would normally be written as:

ドアはその瞬間に開いた。

The spaces here are just a learning aid to make the parts easier to see.

ドア is a loanword, from English door, so it is written in katakana.

Katakana is often used for:

• foreign loanwords
• onomatopoeia
• emphasis
• some technical terms

So ドア is the standard way to write door in Japanese.

は marks the topic of the sentence.

So ドアは means something like:

• as for the door
• the door, ...

The sentence is telling you what happened with the door: it opened at that moment.

This does not necessarily mean strong contrast by itself. Often it just introduces the thing being talked about.

You could say ドアがその瞬間に開いた too, but the nuance changes a little.

A simple way to think about it:

• ドアはその瞬間に開いた。
  = talking about the door as the topic

• ドアがその瞬間に開いた。
  = puts more focus on the door as the subject, or on the fact that it was the door that opened

So は is more topic-oriented, while が is more subject/focus-oriented.

In many situations, both are grammatically fine.

その means that.

It points to something already known from the context. So その瞬間 means:

• that moment
• the moment in question

It suggests the speaker and listener already know which moment is being referred to, or it has been mentioned earlier.

瞬間 means moment or instant.

It usually refers to a very short point in time.

So:

• その瞬間 = that moment
• その瞬間に = at that moment

Here, に marks a specific point in time.

So その瞬間に means:

• at that moment
• at that instant

Japanese often uses に with precise time points, especially when something happens at a particular moment.

In this sentence, it shows that the opening happened exactly then.

Yes, often it can.

For example:

• ドアはその瞬間に開いた。
• ドアはその瞬間、開いた。

Both are natural.

Using に makes the time relation a bit more explicit: at that exact moment.

Without に, it can still sound perfectly natural, especially in narrative writing.

This is a very important distinction.

• 開く（あく） = to open / to become open
  → intransitive
• 開ける（あける） = to open something
  → transitive

So:

• ドアが開いた = the door opened
• 誰かがドアを開けた = someone opened the door

In your sentence, the door is the thing that changed state, so 開いた is the correct choice.

Not necessarily.

開いた only tells you that the door opened. It does not say who or what caused it.

Depending on context, the door may have:

• opened by itself
• been pushed open by someone
• been blown open by the wind
• been opened automatically

Japanese often leaves the cause unstated unless it matters.

The dictionary form is 開く, read あく.

In this sentence:

• 開いた is read あいた
• it is the past tense form of 開く

So:

• 開く → 開いた
• あく → あいた

This is an intransitive verb meaning to open or to become open.

開いた is the plain past form, so it describes a completed event:

• it opened
• it has opened (depending on context)

In this sentence, the opening happened at a specific moment in the past, so the past tense is natural.

Japanese plain past is often used in narration to state that an event occurred.

Not completely. Japanese word order is fairly flexible as long as the particles make the roles clear.

For example, these are all possible:

• ドアはその瞬間に開いた。
• その瞬間にドアは開いた。
• その瞬間、ドアは開いた。

The most important thing is that:

• は marks the topic
• に marks the time point

The verb usually comes at the end.

Yes, it is natural.

It sounds like a straightforward narrative sentence:
The door opened at that moment.

Very natural variations include:

• ドアはその瞬間に開いた。
• その瞬間、ドアは開いた。
• ドアがその瞬間に開いた。

The exact choice depends on what the speaker wants to emphasize.