ame ga furihajimeta node, chikaku no mise ni hairimashita.

In 雨が降り始めた, が marks 雨 as the thing that is doing the action of falling.

With weather expressions, が is very common because it simply introduces what is happening:

• 雨が降る = rain falls / it rains
• 雪が降る = snow falls / it snows

If you used は instead, it would sound more contrastive or topic-based, like as for the rain.... That is possible in some contexts, but 雨が降り始めた is the most natural neutral way to say it started raining.

降り始めた is a compound verb:

• 降る = to fall (for rain, snow, etc.)
• 降り = the ます-stem of 降る
• 始める = to begin
• 始めた = began

So:

• 降り始める = to begin to fall
• 降り始めた = began to fall

This pattern is very common in Japanese:

• 読み始める = start reading
• 食べ始める = start eating
• 話し始める = start talking

So 雨が降り始めた literally means the rain began to fall, which is how Japanese says it started raining.

Japanese often uses the past form to show that one event happened before the next event in the sentence.

Here the sequence is:

1. It started raining.
2. Then the speaker went into a nearby shop.

So 降り始めたので means because it had started raining / since it started raining.

It does not necessarily mean the rain had already finished. It just marks the start of the rain as an event that happened before the speaker entered the shop.

ので means because or since. It connects a reason to a result:

• 雨が降り始めたので、近くの店に入りました。 = Because it started raining, I went into a nearby shop.

Compared with から, ので usually sounds:

• a little softer
• a little more explanatory
• sometimes a little more objective or less blunt

Very roughly:

• から = because
• ので = since / because, but often gentler

Both can be correct here, but ので sounds natural for calmly explaining why the speaker did something.

This is normal Japanese sentence structure.

Before connectors like ので, Japanese usually uses the plain form:

• 降り始めたので

Then the main sentence can still end politely:

• 入りました

So it is very natural to mix them this way:

• subordinate clause: plain form
• final clause: polite form

A fully polite version such as 降り始めましたので、入りました is possible, but it sounds more formal or stiff in everyday conversation.

Both are possible, but they are not exactly the same.

• 近くの店 = a shop nearby / a shop in the vicinity
• 近い店 = a shop that is close

In this sentence, 近く acts like a noun meaning nearby area or vicinity, so 近くの店 literally means a shop of the nearby area, or more naturally, a nearby shop.

This is a very common pattern:

• 近くの駅 = a nearby station
• 近くの公園 = a nearby park

So 近くの店 is the most natural phrasing here.

Because 近く is functioning like a noun here, and の connects nouns.

You can think of it like this:

• 近く = vicinity / nearby place
• 店 = shop
• 近くの店 = shop in the vicinity / nearby shop

This is one of the many uses of の: linking one noun to another.

Because the verb 入る usually takes に to mark the place entered.

• 店に入る = enter a shop
• 部屋に入る = enter a room
• 家に入る = enter a house

So 店に入りました means entered the shop or went into the shop.

English uses into, but Japanese uses the particle に with 入る.

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

So 近くの店に入りました does not explicitly say I entered a nearby shop, but that is understood from the situation.

This is very normal in Japanese. If the subject is obvious, it is usually omitted.

Yes. The comma is helpful, but not required.

• 雨が降り始めたので近くの店に入りました。

is also correct.

The comma just makes the break between the reason and the main action easier to read:

• reason: 雨が降り始めたので
• result: 近くの店に入りました

So the comma is a writing choice, not an essential part of the grammar.