sono syunkan, omoidasimasita.

Both can be correct, but they feel slightly different.

• その瞬間 = that moment / at that moment, used a bit more like a time expression on its own
• その瞬間に = explicitly marks the time with に

In Japanese, some time expressions can appear without に, especially when they already clearly function as a time setting. So:

• その瞬間、思い出しました。
• その瞬間に、思い出しました。

Both are natural. The version without に can sound a little smoother or more literary in some contexts.

その means that, usually referring to something close to the listener or something just mentioned in the conversation or text.

Japanese has three main demonstratives:

• この = this
• その = that
• あの = that over there / that we both know

So その瞬間 means that moment, usually a moment that has just been mentioned or is understood from context.

瞬間 means instant or moment.

It refers to a very brief point in time. Compared with 時, which is a more general word for time/when, 瞬間 feels more specific and dramatic: a precise instant.

So その瞬間 has a strong feeling of:

• at that very moment
• the instant that happened

The comma helps separate the time phrase from the main action:

• その瞬間、 = At that moment,
• 思い出しました。 = I remembered.

Japanese commas often mark a pause or make the sentence easier to read. It is especially common after time expressions at the beginning of a sentence.

The comma is helpful, but not always absolutely required:

• その瞬間思い出しました。 is possible
• その瞬間、思い出しました。 is easier to read and more natural in writing

Because the act of remembering happened at a specific point in time.

思い出す means to remember / to recall.
Its past polite form is 思い出しました.

This does not mean the remembered thing itself is in the past. It means the moment of recalling happened in the past.

So the sentence describes a completed event:

• there was a moment
• at that moment, the speaker remembered something

It comes from the dictionary form 思い出す.

Conjugation:

• 思い出す = dictionary form
• 思い出し = stem
• 思い出します = polite non-past
• 思い出しました = polite past

This is a -す verb, so it follows the normal pattern for u-verbs:

• 話す → 話しました
• 直す → 直しました
• 思い出す → 思い出しました

This is a very common question.

• 思い出す = to recall, to bring something back to mind
• 覚えている = to remember, to have something in memory

So:

• その瞬間、思い出しました。 = At that moment, I remembered it / it came back to me.
• 覚えています。 = I remember it / I know it and still retain it.

A useful way to think about it:

• 思い出す focuses on the moment of remembering
• 覚えている focuses on the state of already remembering

Because Japanese often omits the subject when it is clear from context.

In English, you usually need to say I remembered.
In Japanese, if it is obvious who is doing the action, you can simply say:

• その瞬間、思い出しました。

The listener will often understand the subject as:

• I
• sometimes we
• or whoever the context is about

This omission is extremely common and natural in Japanese.

Japanese often leaves out the object too if it is understood from context.

A fuller sentence could be:

• その瞬間、名前を思い出しました。
  At that moment, I remembered the name.

But if everyone already knows what is being talked about, Japanese often drops it:

• その瞬間、思い出しました。

So the sentence really means something like:

• At that moment, I remembered it.
• At that moment, it came back to me.

ました makes the sentence polite.

Compare:

• 思い出した。 = plain past
• 思い出しました。 = polite past

The polite form is common in:

• conversation with people you are not close to
• formal narration
• textbook examples
• customer-facing or respectful speech

The plain form would sound more casual, direct, or story-like depending on context.

Historically and structurally, yes, you can see it as:

• 思い from 思う = think
• 出す = put out / bring out / start to

So 思い出す gives the idea of bringing something out from your thoughts/memory, which matches to recall very well.

However, in actual usage, you should learn 思い出す as a single verb meaning:

• to remember
• to recall

You do not usually need to analyze it every time you see it.

It is read しゅんかん.

So the full sentence is:

• そのしゅんかん、おもいだしました。

Readings:

• その = sono
• 瞬間 = shunkan
• 思い出しました = omoidasimashita / more accurately omoidashimashita

Note that in natural pronunciation, 思い出しました can sound a bit fast and smooth, but the standard reading is おもいだしました.