bogo sipeossdeon yeonghwaga byeolloyeoseo jom silmanghaesseo.

It is built like this:

• 보다 = to see/watch
• 보고 싶다 = to want to see/watch
• 보고 싶었던 = wanted to see/watch as a noun-modifying form

So 보고 싶었던 영화 means the movie I had wanted to watch.

The -었던 part makes the wanting feel like a past state. In other words, before actually watching the movie, the speaker had already wanted to see it.

Because 영화 is the subject of the main descriptive part, 별로여서.

A helpful way to see the structure is:

• [보고 싶었던] 영화가
• 별로여서
• 좀 실망했어

Inside 보고 싶었던, the movie is the thing being watched, but that role is already built into the relative clause. After that, 영화 becomes the subject of 별로여서, so it takes 가.

Very literally, the structure is closer to:

• The movie that I had wanted to watch
  • wasn’t that great
    • so I was a bit disappointed

Because Korean often leaves out subjects when they are obvious from context.

Here, the speaker is understood in two places:

• in 보고 싶었던: (I) had wanted to watch
• in 실망했어: (I) was disappointed

So even without 나는 or 내가, the meaning is clear.

Here, 별로 means something like:

• not that great
• so-so
• underwhelming
• not particularly good

Many learners first meet 별로 in patterns like 별로 안... or 별로 못..., where it goes with a negative expression. But in everyday speech, people also say things like:

• 그 영화 별로야
• 음식이 별로였어

In this sentence, 별로여서 means because it wasn’t that good / because it was kind of underwhelming.

-여서 here connects two ideas and gives a reason, like:

• because
• so

So:

• 영화가 별로여서 좀 실망했어
• The movie wasn’t that great, so I was a bit disappointed

The sentence is showing cause and result:

• cause: the movie was underwhelming
• result: I felt disappointed

좀 literally can mean a little, but in sentences like this it often softens the statement.

So 좀 실망했어 can mean:

• I was a little disappointed
• I was kind of disappointed
• I was somewhat disappointed

It makes the sentence sound less blunt and a bit more natural in conversation.

Because the speaker is talking about their own feeling, not directly describing the movie.

• 실망했어 = I was disappointed
• 실망스러웠어 = it was disappointing

So these are different in focus:

• 좀 실망했어 focuses on my reaction
• 좀 실망스러웠어 focuses on the movie itself

Both can work in similar situations, but they are not exactly the same.

Because 싶었던 shows that the desire existed in the past.

Compare:

• 보고 싶은 영화 = a movie I want to watch
• 보고 싶었던 영화 = a movie I had wanted to watch

In this sentence, the speaker has already watched the movie and is now reflecting on it. So 싶었던 fits better, because the wanting happened before the disappointment.

They are similar, but there is a nuance difference.

• 보고 싶었던 영화 more clearly points to a past desire that existed before and is now being looked back on
• 보고 싶던 영화 can sound a little more like recalling an ongoing past desire or a remembered state

In this sentence, 보고 싶었던 영화 is very natural because the speaker is talking about a movie they had wanted to watch, then finally watched, and then felt disappointed.

실망했어 is casual, plain speech. It is used with friends, people close to you, or in informal conversation.

The polite version would be:

• 보고 싶었던 영화가 별로여서 좀 실망했어요.

The meaning is the same. The difference is just speech level.

Not very naturally. A more literal breakdown is:

• 보고 싶었던 = had wanted to watch
• 영화가 = the movie
• 별로여서 = because it was not that great / so-so
• 좀 실망했어 = I was a bit disappointed

A natural English rendering would usually be something like:

• The movie I’d wanted to watch wasn’t that great, so I was a bit disappointed.

So the grammar lines up well, but a fully word-for-word translation sounds awkward in English.