raisyuu tomodati ga kaigai ni iku node, kuukou he miokuri ni ikimasu.

Because 友達が海外に行く is the reason clause: my friend is going abroad.

Here, が marks who is doing the action of 行く. In other words:

• 友達が行く = my friend is the one who is going

This is very natural before ので.

If you used は instead, the nuance would change. 友達は would make friend more of a topic or contrast, while が simply identifies the subject of that clause.

Many time expressions in Japanese can be used without a particle.

So:

• 来週行く = go next week

This is normal. In fact, adding に after 来週 would usually sound unnatural.

A rough rule:

• Words like 来週, 今日, 明日, 昨日 often do not take に
• More specific time points like 3時, 月曜日, 7月1日 often can take に

So 来週 友達が海外に行くので… is perfectly natural.

ので means because / since.

In this sentence, it connects:

• 来週友達が海外に行く = my friend is going abroad next week
• 空港へ見送りに行きます = I’m going to the airport to see them off

So the whole thing is:

• Because my friend is going abroad next week, I’m going to the airport to see them off.

Compared with から, ので often sounds:

• a bit softer
• a bit more explanatory
• a bit less direct

So ので is very natural in a sentence like this, where the first part calmly gives the reason for the second.

Because に marks the destination with verbs of movement like 行く.

So:

• 海外に行く = go abroad

Here, 海外 means overseas / abroad, and に shows that it is the place or destination the person is going to.

You could think of it as:

• X に 行く = go to X

Even though 海外 is not a specific country, it still works as a destination in Japanese.

Both へ and に can be used with 行く.

So both of these are possible:

• 空港へ行く
• 空港に行く

The difference is small:

• へ emphasizes the direction toward a place
• に emphasizes the destination / arrival point

In many everyday sentences, the difference is very slight, and either one may sound natural. Here, 空港へ is a very normal choice.

This is the pattern:

• noun + に + 行く = go in order to do something

Here, 見送り is a noun meaning seeing someone off / send-off.

So:

• 見送りに行きます = go to see someone off

This is a very common pattern in Japanese:

• 買い物に行く = go shopping
• 食べに行く = go eat
• 迎えに行く = go pick someone up
• 見送りに行く = go see someone off

So the に after 見送り is not marking a destination. It marks the purpose of going.

Yes. 見送り is the noun form of 見送る.

• 見送る = to see someone off
• 見送り = seeing someone off / send-off

So the sentence could be understood as using the noun form to fit the pattern:

• 見送りに行く = go for a send-off / go to see someone off

This is very natural Japanese.

Also, even though 見送る looks like 見る + 送る, it functions as its own verb meaning to see off, not literally look + send in this sentence.

Because there are two separate actions in the sentence:

1. 友達が海外に行く
   → my friend is going abroad

2. 空港へ見送りに行きます
   → I am going to the airport to see them off

So the first 行く is about the friend, and the second 行きます is about the speaker.

Japanese does not avoid repeating a verb just because English might try to. Repeating 行く here is completely normal.

The person going to the airport is understood to be the speaker, even though 私は is omitted.

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

So the full idea is something like:

• 私は、来週友達が海外に行くので、空港へ見送りに行きます。

But in natural Japanese, 私は is often unnecessary.

This is very common:

• if the speaker is obviously talking about their own action,
• Japanese usually does not repeat I.

友達 can mean friend or friends, depending on context. It does not show gender either.

So this sentence could refer to:

• a friend
• some friends

and it does not tell you whether the friend is male or female.

In many textbook translations, it may be rendered as my friend because that is the simplest reading, but the Japanese itself is more flexible.

The comma simply helps show a pause between the reason and the main action:

• 来週友達が海外に行くので、
• 空港へ見送りに行きます。

It makes the sentence easier to read, especially when the first part is a reason clause.

The comma is natural here, but punctuation in Japanese can be somewhat flexible. The sentence would still be understandable without it.