tomodati ga buzi ni ie ni tuita to kiite, ansinsimasita.

In this sentence, 友達が marks friend as the subject of the quoted information:

• 友達が無事に家に着いた = my friend arrived home safely

Using が here is very natural because it presents the information inside the と聞いて clause in a neutral way.

If you used は instead, it could sound more contrastive or as if friend were already the main topic:

• 友達は無事に家に着いたと聞いて...

That is possible in some contexts, but が is the more straightforward choice here.

They do two different jobs:

1. 無事に

   • Here, に turns 無事 into an adverb-like expression.
   • 無事に means safely, without trouble, or without incident.

2. 家に

   • Here, に marks the destination of 着く.
   • 家に着く = arrive at home

So even though both are に, they are not doing the same thing.

無事 means something like:

• safety
• being unharmed
• without problems

When you add に, it becomes adverbial:

• 無事に着いた = arrived safely
• 無事に終わった = finished safely / without problems

So in this sentence, 無事に describes how the friend arrived home.

Both can be natural, but they emphasize slightly different things:

• 着く = to arrive
• 帰る = to return / go back

So:

• 家に着いた focuses on the moment of arrival
• 家に帰った focuses more on the act of going back home

Since the sentence is about the relief of hearing that the friend made it home safely, 着いた works very well because it highlights successful arrival.

Because the arrival happened before the speaker heard about it.

The sequence is:

1. The friend arrived home safely.
2. The speaker heard that.
3. The speaker felt relieved.

So the quoted event is completed, which is why 着いた is past tense.

と marks the content that was heard, and 聞いて is the -て form of 聞く.

So:

• 友達が無事に家に着いたと聞いて = hearing that my friend arrived home safely

This -て form connects to the next clause:

• 安心しました = I felt relieved

So the whole thing means:

• Hearing that my friend arrived home safely, I felt relieved.

It gives the reason or background for the feeling of relief.

Here it means hear.

聞く can mean both to hear and to ask, depending on context. But with a quoted clause marked by と, it very commonly means:

• hear that ...

So:

• 〜と聞いて = hearing that ...

If it meant ask, the structure would usually be different, often involving the person being asked:

• 友達に聞く = ask a friend

Because the speaker is describing a completed emotional reaction:

• I felt relieved

In Japanese, feelings in a narrative like this are often put in the past when the speaker is talking about what happened at that time.

• 安心しました = polite past
• plain form: 安心した

If you said 安心します here, it would not fit this completed sequence of events.

The subject is understood from context as I.

Japanese often omits subjects when they are obvious. So instead of saying:

• 私は安心しました

it is very natural to simply say:

• 安心しました

Because the sentence is about the speaker’s reaction, I is understood automatically.

Yes. 友達 can be singular or plural depending on context.

So this could mean:

• my friend
• my friends

In many cases, English requires you to choose singular or plural, but Japanese often leaves that unspecified unless it matters.

If someone wanted to make it clearly plural, they might say:

• 友達たち

But in ordinary Japanese, just 友達 is very common.

The と here is a quotation particle.

It marks the content of what was heard:

• 友達が無事に家に着いた = the content
• と聞いて = hearing that ...

So と is similar to putting the previous clause into an English that-clause:

• I heard that my friend arrived home safely

This is a very common pattern with verbs like:

• 言う = say
• 思う = think
• 聞く = hear

The comma simply shows a pause and makes the sentence easier to read.

• 友達が無事に家に着いたと聞いて、安心しました。

It separates the background/reason part from the main result:

• hearing that my friend arrived home safely / I felt relieved

It is helpful, but in many cases Japanese punctuation is more flexible than English punctuation. The sentence would still be understandable without the comma.