oneureun jungyohan hoeuiga isseoseo sae syeocheureul ibeosseoyo.

은/는 is the topic marker, so 오늘은 means something like as for today or today, .... It sets the scene for the sentence.

Using 오늘이 would mark today as the grammatical subject, which does not sound natural here. The sentence is really about what I did today, so 오늘은 works much better.

• 오늘은 = as for today / today
• natural nuance: Today, since I have an important meeting, I wore a new shirt.

Because the sentence uses 있다 in the phrase 회의가 있다, which means to have a meeting or literally a meeting exists.

In Korean, with 있다, the thing that exists is usually marked with 이/가, not 을/를.

So:

• 회의가 있다 = there is a meeting / I have a meeting
• 회의를 있다 = incorrect

That is why you get:

• 중요한 회의가 있어서 = because there is / because I have an important meeting

있어서 is from 있다 + -어서.

Here, -어서 connects two clauses and gives a reason, so 회의가 있어서 means:

• because I have a meeting
• since there is a meeting

So the sentence structure is:

• 오늘은 중요한 회의가 있어서 = Because I have an important meeting today,
• 새 셔츠를 입었어요 = I wore/put on a new shirt.

A very literal breakdown is:

• 회의가 있다 = there is a meeting
• 회의가 있어서 = because there is a meeting

중요하다 is the dictionary form meaning to be important.
When a descriptive verb modifies a noun, it changes into an attributive form.

So:

• 중요하다 = to be important
• 중요한 = important (before a noun)

That is why:

• 중요한 회의 = an important meeting

You cannot say 중요하다 회의 in normal Korean, because the dictionary form does not directly go in front of a noun.

Yes, 새 means new when it comes directly before a noun.

So:

• 새 셔츠 = a new shirt

This is a very common word, but it can confuse learners because Korean also has 새롭다. Related forms include:

• 새 = new (directly before a noun)
• 새로운 = new / novel

For everyday things like new shirt, new car, new bag, 새 + noun is very common:

• 새 옷 = new clothes
• 새 차 = new car
• 새 가방 = new bag

So 새 셔츠 is completely natural.

Because 입다 means to wear / put on, and the thing being worn takes the object particle 을/를.

So:

• 셔츠를 입다 = to wear a shirt
• 옷을 입다 = to wear clothes
• 정장을 입다 = to wear a suit

Here:

• 새 셔츠를 = a new shirt as the object
• 입었어요 = wore / put on

Because 입었어요 describes the action of putting on / wearing as something the speaker already did.

Even if the meeting is today, the act of putting on the shirt happened before the moment of speaking, so the past tense is natural.

Compare:

• 입어요 = I wear / I put on / I will wear (depending on context)
• 입었어요 = I wore / I put on

In this sentence, the idea is:

• I had an important meeting today, so I put on/wore a new shirt.

If you wanted to emphasize the current state of wearing it right now, Korean often uses:

• 입고 있어요 = I am wearing it

But in this sentence, 입었어요 is the normal choice.

It is omitted because Korean very often leaves out subjects when they are obvious from context.

In English, you usually need to say I:

• I wore a new shirt.

In Korean, if it is clear who is speaking, you can simply say:

• 새 셔츠를 입었어요.

So the full sentence naturally implies:

• (저는) 오늘은 중요한 회의가 있어서 새 셔츠를 입었어요.
• (As for me,) today, because I had an important meeting, I wore a new shirt.

Omitting the subject is extremely common and natural.

Yes, you could, but the nuance and style are a little different.

Original:

• 중요한 회의가 있어서 = because I have an important meeting

Alternative:

• 중요한 회의 때문에 = because of the important meeting

Both can work, but -아서/어서 often feels smoother and more conversational when linking two actions or situations in one sentence.

So:

• 회의가 있어서 새 셔츠를 입었어요.
  = natural, everyday Because I had a meeting, I wore a new shirt.

• 회의 때문에 새 셔츠를 입었어요.
  = also possible, more like Because of the meeting, I wore a new shirt.

Korean usually puts information in this general order:

time/topic + reason/background + object + verb

So this sentence breaks down like this:

• 오늘은 = today / as for today
• 중요한 회의가 있어서 = because I have an important meeting
• 새 셔츠를 = a new shirt (object)
• 입었어요 = wore / put on

So the overall structure is:

Today, because I have an important meeting, I wore a new shirt.

This is very typical Korean sentence order: the main verb comes at the end.

It can be understood as either, depending on context.

The verb 입다 can refer to:

• putting on clothes
• wearing clothes

In many sentences, English chooses one or the other based on what sounds most natural.

Here, because there is a reason for the action (because I have an important meeting), English often translates it as:

• I wore a new shirt or
• I put on a new shirt

Both are reasonable. The Korean sentence itself does not always force a sharp distinction in the way English sometimes does.