uni johaseo jijeongseogeul sasseo.

Because 좋다 is a descriptive verb/adjective, and Korean usually marks the thing being described with 이/가.

• 운이 좋다 = luck is good → to be lucky
• So 운이 is the natural pattern.
• 운을 좋다 is not grammatical here.

This is a very common Korean structure:

• 날씨가 좋다 = the weather is good
• 기분이 좋다 = the mood/feeling is good
• 운이 좋다 = luck is good

Here, -아서 / -어서 connects two clauses and gives a sense of:

• because
• so
• sometimes and, when the first clause is background information

So 운이 좋아서 지정석을 샀어 literally feels like:

• Because I was lucky, I bought a reserved seat
• or more naturally, I was lucky, so I got/bought a reserved-seat ticket

In many contexts, 운이 좋아서 can sound a lot like luckily.

It is the stem 좋- plus -아서.

Step by step:

• dictionary form: 좋다
• stem: 좋-
• add -아서
• result: 좋아서

Korean uses -아/어서 after the verb/adjective stem, and the choice between -아서 and -어서 depends on the vowel in the stem. Since 좋- contains ㅗ, it takes -아서.

지정석 means a reserved seat or assigned seating.

It is made of:

• 지정 = designation / assignment
• 석 = seat

Depending on context, 지정석 can refer to:

• the actual reserved seat
• a reserved-seat ticket
• the reserved-seat option/class

So in real life, 지정석을 샀어 often means I bought a reserved-seat ticket, even though the word ticket is not explicitly stated.

을/를 marks the direct object of the verb.

Here:

• 지정석을 = the thing being bought
• 샀어 = bought

So 지정석을 샀어 means bought a reserved seat / reserved-seat ticket.

Even though English speakers may expect ticket to be the object, Korean often just uses 지정석 as shorthand.

샀어 is the past casual form of 사다 (to buy).

Step by step:

• dictionary form: 사다
• stem: 사-
• past marker: -았-
• 사았어 contracts to 샀어

So:

• 사다 = to buy
• 샀어 = bought

This is casual, non-polite speech. The polite version would be:

• 샀어요

Yes. Korean often leaves out the subject when it is obvious from context.

So this sentence could really be understood as:

• 나는 운이 좋아서 지정석을 샀어
• 제가 운이 좋아서 지정석을 샀어요

But Korean usually does not say 나/저 unless it is needed for emphasis or clarity.

That is very normal. In conversation, people often say just:

• 운이 좋아서 지정석을 샀어

and the listener understands who did it from context.

It can feel like either, depending on context.

Grammatically, 운이 좋아서 means because I was lucky. But in natural English, that often comes across more like:

• I was lucky and got a reserved seat
• Luckily, I got a reserved-seat ticket

That is because luck here usually refers to favorable circumstances, such as:

• there was one seat left
• someone canceled
• the timing worked out well

So the Korean sentence may sound a bit more natural in English if translated with luckily or I was lucky enough to..., even though the Korean literally uses a cause-and-effect structure.

It could, but Korean often leaves out predictable nouns when the meaning is clear.

For example:

• 지정석 표를 샀어 = I bought a reserved-seat ticket
• 지정석을 샀어 = I bought a reserved seat / reserved-seat ticket

Both can work, but the shorter version is common when the situation already makes it obvious that we are talking about tickets.

This kind of omission is very common in Korean and can sound completely natural.

It is pronounced roughly like 사써.

Why:

• the written form is 샀어
• the final ㅆ links into the following vowel 어
• in actual pronunciation, it sounds like 싸/써 across the syllables

So a natural pronunciation is close to:

• 운이 좋아서 지정서글 사써

That is not the spelling, just a rough pronunciation guide.