ooame no keihou ga deta node, kaeru zikan wo hayamemasita.

の links the two nouns and shows what kind of warning it is.

• 大雨 = heavy rain
• 警報 = warning, alarm

So 大雨の警報 means a warning for heavy rain or a heavy-rain warning.

In Japanese, noun + の + noun is a very common pattern. It can show:

• possession
• category
• content
• relationship

Here it marks category/type: what warning? → the heavy-rain warning.

Also, in real-life Japanese, you may also see 大雨警報 without の, especially in official weather language.

が marks 警報 as the thing that appeared / was issued.

The verb here is 出た, from 出る, and in this kind of expression, what comes out or is issued is marked with が.

So:

• 警報が出た = a warning was issued

This is natural because 出る is an intransitive verb. The warning is the thing that occurs or appears, so it takes が, not を.

This is an idiomatic use of 出る.

The basic meaning of 出る is to go out / to come out / to appear, but Japanese often uses it for announcements, orders, warnings, and results:

• 警報が出る = a warning is issued
• 命令が出る = an order is given
• 結果が出る = results come out

So 警報が出た does not mean the warning physically moved somewhere. It means the warning was announced/issued.

This is a very common pattern to memorize as a chunk:

• 〜が出る
• especially with notices, warnings, official information, and results

Because the warning had already been issued before the speaker changed their return time.

The sentence describes this sequence:

1. The heavy-rain warning came out / was issued
2. Because of that, I moved my return time earlier

So 出た is past because it is a completed event that became the reason for the next action.

Japanese often uses past tense in subordinate clauses like this when the earlier event is already settled:

• 雨が降ったので、行きませんでした。
• 電車が遅れたので、遅刻しました。

ので gives a reason or cause: because, since.

So:

• 警報が出たので = because a warning was issued

Compared with から, ので usually sounds:

• a bit softer
• a bit more objective
• a bit more explanatory

Very roughly:

• から can sound more direct or personal
• ので can sound more neutral or matter-of-fact

In this sentence, ので fits well because the speaker is explaining a decision based on circumstances.

Compare:

• 警報が出たので、帰る時間を早めました。
  Because a warning was issued, I moved my return time earlier.

• 警報が出たから、帰る時間を早めました。
  Also correct, but a little more direct/conversational.

Because 帰る時間 means the time to go home / the time when I go home, not the time when I went home.

In Japanese, a plain-form verb before a noun works like a relative clause.

• 帰る時間 = the time of returning / the time to return
• 帰った時間 = the time when someone returned

Here the speaker is talking about a planned or chosen return time, so 帰る is the natural form.

This is a very important pattern:

• 食べる時間 = time to eat
• 寝る時間 = time to sleep
• 出発する時間 = departure time / time to leave

It refers to the time of going home or the planned time to leave and return home.

It does not necessarily mean the exact moment of arriving home. In many contexts, it means something like:

• the time to head home
• when I planned to leave for home
• my return-home time

Japanese often leaves this kind of detail to context.

So 帰る時間を早めました means the speaker decided to go home earlier than originally planned.

Because 時間 is the object of 早めました.

The verb 早める means to make earlier, to advance, or to move up. The thing being made earlier is marked with を.

So:

• 時間を早める = to make the time earlier
• 出発を早める = to move the departure earlier
• 予定を早める = to move the schedule up

That is why 時間 takes を here.

This is a transitive vs. intransitive pair.

• 早める = to make something earlier (transitive)
• 早まる = something becomes earlier (intransitive)

In this sentence, the speaker actively changed the time, so 早めました is used.

Compare:

• 帰る時間を早めました。
  I made my return time earlier.

• 帰る時間が早まりました。
  My return time became earlier.

The first focuses on the speaker’s action.
The second focuses on the change itself.

Sometimes Japanese learners try that, but 早めました is more natural here.

Why?

• 早い = early
• 早くする = to make something early / to do earlier
• 早める = to move something earlier, often by some amount

Both can sometimes work, but 時間を早める is a very common and natural collocation for schedules and timing.

So in this sentence, 早めました sounds smoother and more idiomatic.

Because Japanese often omits subjects when they are clear from context.

In 帰る時間を早めました, the person who changed the time is understood from the situation. In many cases, it is naturally interpreted as:

• I moved my return time earlier or
• we moved our return time earlier

Japanese does this all the time. If the subject is obvious, it is usually left unstated.

That does not make the sentence vague to a native speaker; context normally supplies the missing subject.

This is normal Japanese sentence structure.

Before ので, the clause uses the plain form:

• 出たので

The main sentence ends politely:

• 早めました

This is a very common pattern:

• subordinate clause = plain form
• sentence ending = polite form

For example:

• 雨が降ったので、行きませんでした。
• 時間がないので、急ぎます。

So the politeness of the whole sentence is mainly shown at the end.

No, it is not strictly necessary, but it is very common.

The comma helps separate:

• the reason part: 大雨の警報が出たので
• the main action: 帰る時間を早めました

So it improves readability, especially in a sentence with a clear cause-and-result structure.

You could also write:

• 大雨の警報が出たので帰る時間を早めました。

That is still correct.

They are very close, but the nuance and style are slightly different.

• 大雨の警報 = a warning for heavy rain
  More general descriptive phrasing.

• 大雨警報 = heavy rain warning
  This is the official weather-term style, more compact and technical.

In everyday explanation, both can be understood. In weather reports and official announcements, 大雨警報 is especially common.

So the sentence with 大雨の警報 is natural, but in real weather-related Japanese you may often see 大雨警報が出た.