chinguga naeil ondaneun sosigeul deutgo jeongmal gippeosseo.

A natural breakdown is:

• 친구가 = my friend
  • subject marker 가
• 내일 = tomorrow
• 온다는 = that (someone) is coming
• 소식을 = the news
  • object marker 을
• 듣고 = hearing / after hearing
• 정말 = really
• 기뻤어 = was happy

So the structure is basically:

[friend is coming tomorrow] + news + hearing + really + was happy

A natural English translation is:

I was really happy to hear the news that my friend is coming tomorrow.

The subject I is not stated, but it is understood from context.

Here, 친구가 marks friend as the subject inside the embedded clause:

• 친구가 내일 온다 = My friend is coming tomorrow

Using 가 is very natural because the sentence is presenting the information that it is the friend who is coming.

If you used 친구는, it would sound more like setting up friend as a topic, with a contrastive or topical feel:

• 친구는 내일 온다는 소식... = As for my friend, the news that they are coming tomorrow...

That is possible in some contexts, but 친구가 is the neutral and most natural choice here.

온다는 is a quoted/adnominal form built from:

• 오다 = to come
• 온다 = comes / is coming
• -는 attached to the quoted form -다고 하다, compressed into -다는 before a noun

So:

• 친구가 내일 온다는 소식 literally means
  the news saying that my friend is coming tomorrow

This pattern is very common:

• 비가 온다는 소식 = news that it will rain
• 시험이 어렵다는 이야기 = a story that the exam is hard
• 그가 떠난다는 말 = the words/news that he is leaving

So -다는 here connects a clause to a noun like 소식, 말, 이야기, 사실, etc.

This is a very common question.

In Korean, the plain present form often covers scheduled or expected future events, especially when there is a time word like 내일.

So:

• 친구가 내일 온다 = My friend is coming tomorrow
• 내일 비가 온다 = It’s going to rain tomorrow

Because 내일 already tells you the time, 온다 can naturally refer to a future event.

You could also say:

• 친구가 내일 올 거라는 소식 = the news that my friend will come tomorrow

That is also grammatical, but it sounds a bit more explicitly future-oriented.
온다는 소식 is very natural and common.

소식 means news, word, or tidings.

In this sentence:

• 친구가 내일 온다는 소식 = the news that my friend is coming tomorrow

So the speaker is not directly saying:

• I heard my friend is coming tomorrow

Instead, the sentence is framed as:

• I heard the news that my friend is coming tomorrow

This gives the sentence a slightly more noun-based structure.

A very similar sentence would be:

• 친구가 내일 온다고 듣고 정말 기뻤어.

That means nearly the same thing:
I was really happy to hear that my friend is coming tomorrow.

The version with 소식 sounds a bit more like hearing the news.

In many beginner glosses, 듣다 is given as to listen, but in actual Korean it often covers both:

• to listen
• to hear
• to be told / to learn by hearing

So:

• 소식을 듣다 = to hear the news
• 이야기를 듣다 = to hear a story / hear what someone says
• 그 말을 듣고 놀랐어 = I was surprised to hear that

In this sentence, 듣고 is best translated as hearing or after hearing.

This is because 듣다 is a ㄷ-irregular verb, but the irregular change only happens when the following ending begins with a vowel.

Examples:

• 듣다 + 어요 → 들어요
• 듣다 + 으면 → 들으면

But -고 begins with a consonant, so the stem stays as 듣-:

• 듣다 + 고 → 듣고

So 듣고 is exactly what you should expect here.

Here -고 connects two actions or states:

• 소식을 듣고 = hearing the news / after hearing the news
• 정말 기뻤어 = I was really happy

In this sentence, -고 has a natural after doing X or upon doing X feeling.

So the meaning is:

• After hearing the news, I was really happy
• I was really happy when I heard the news

Sometimes -고 is just sequential (and then), but here it also implies the emotional reaction happened because of the first event.

This is about how Korean expresses emotion adjectives.

• 기쁘다 = to be glad / happy
• 기뻤어 = was happy / felt happy

This is the most natural way to describe your own emotional state.

기뻐하다 is more like to show happiness or to act happy, and it is often used for other people, because you are describing their observable behavior.

So:

• 나는 정말 기뻤어 = I was really happy
• 그는 정말 기뻐했어 = He looked really happy / He was expressing happiness

For the speaker’s own feelings, 기뻤어 is the normal choice.

The subject of 기뻤어 is understood to be the speaker: I.

Korean very often omits subjects when they are obvious from context. So instead of saying:

• 나는 친구가 내일 온다는 소식을 듣고 정말 기뻤어

Korean naturally drops 나는 if it is already clear.

That is why the sentence sounds natural without an explicit I.

정말 is an adverb meaning really, truly, or very much depending on context.

Here it intensifies 기뻤어:

• 정말 기뻤어 = I was really happy

It is very common in spoken Korean.

Similar examples:

• 정말 좋았어 = It was really good
• 정말 놀랐어 = I was really surprised
• 정말 반가웠어 = I was really glad to see you

기뻤어 is in the casual/informal polite-neutral-to-close relationship style, often used with friends, family, diaries, or casual narration.

Compare:

• 기뻤어 = casual
• 기뻤어요 = polite
• 기뻤다 = plain/written style

So the speaker is using a casual speech level here.
If you wanted to make it polite, you could say:

• 친구가 내일 온다는 소식을 듣고 정말 기뻤어요.

Yes. A very common simpler version is:

• 친구가 내일 온다고 듣고 정말 기뻤어.

This means almost the same thing:

• I was really happy to hear that my friend is coming tomorrow.

The difference is mostly structural:

• 온다는 소식을 듣고 = hearing the news that...
• 온다고 듣고 = hearing that...

Both are natural.
The original sentence sounds slightly fuller and more like hearing the news.

내일 modifies 오다.

So the meaning is:

• 친구가 내일 온다 = My friend comes/is coming tomorrow

It does not mean something like tomorrow’s friend or anything attached to 친구.

In Korean, time expressions like 오늘, 내일, 어제 often come before the verb phrase they belong to.

Yes, it is very useful to learn this as part of a broader pattern:

• [clause] + 다는 + noun

especially with nouns like:

• 소식 = news
• 말 = statement / words
• 이야기 = story / talk
• 사실 = fact
• 생각 = thought/idea

Examples:

• 그가 결혼한다는 소식 = the news that he is getting married
• 시험이 어렵다는 말 = the statement that the test is difficult
• 그 사람이 온다는 이야기를 들었어 = I heard that that person is coming

So yes, this sentence contains a very common and useful grammar pattern.