i munjeneun saenggakboda swiwoyo.

는 is the topic marker. It marks 이 문제 (this problem) as the thing the speaker is talking about.

So 이 문제는 means something like:

• as for this problem
• this problem, ...

In this sentence, it sets up the topic and then comments on it: it’s easier than expected.

A very natural breakdown is:

• 이 문제는 = as for this problem
• 생각보다 쉬워요 = it is easier than one thought / easier than expected

Because 문제 is not the object of a verb here. It is the thing being described.

The sentence is not saying do this problem or solve this problem. It is saying that this problem is easy.

So:

• 이 문제는 = as for this problem
• 이 문제를 would mean this problem as the object of an action

For example:

• 이 문제는 쉬워요. = This problem is easy.
• 이 문제를 풀어요. = I solve this problem.

생각보다 means than expected, than I thought, or more literally than thought.

It is made from:

• 생각 = thought, thinking, expectation
• 보다 = than, compared with

So 생각보다 쉬워요 literally means:

• It is easy compared to what I thought
• more naturally: It’s easier than I thought

This is a very common expression in Korean.

Examples:

• 생각보다 비싸요. = It’s more expensive than I thought.
• 생각보다 어려워요. = It’s harder than I thought.
• 생각보다 빨라요. = It’s faster than I thought.

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

In English, you usually say:

• This problem is easier than I thought.

In Korean, the I is often omitted because it is understood. So 생각보다 naturally carries the idea of:

• than I thought
• than we thought
• than expected

depending on the situation.

If you want to say it more fully, you could say something like:

• 제가 생각한 것보다 쉬워요. = It’s easier than I thought.

But 생각보다 쉬워요 is shorter and very natural in everyday Korean.

Because the dictionary form is 쉽다 and this is a ㅂ-irregular adjective.

When 쉽다 combines with -어요, the ㅂ changes and you get:

• 쉽다 → 쉬워요

So:

• 쉽다 = to be easy
• 쉬워요 = is easy / it’s easy

This same pattern happens with some other words too, such as:

• 어렵다 → 어려워요
• 덥다 → 더워요
• 춥다 → 추워요

So 쉬워요 is the correct polite form.

쉬워요 is in the polite informal style, often called the -요 form.

It is very common in everyday conversation and is appropriate in many situations.

Compare:

• 쉬워. = casual/plain conversational style
• 쉬워요. = polite everyday style
• 쉽습니다. = formal polite style

So 이 문제는 생각보다 쉬워요. sounds polite and natural for normal conversation.

Yes, you could, and it would also be natural.

The difference is mostly about focus:

• 이 문제는 생각보다 쉬워요.
  As for this problem, it’s easier than expected.
  This feels more like introducing this problem as the topic.

• 이 문제가 생각보다 쉬워요.
  This problem is easier than expected.
  This can feel a bit more like identifying which thing is easy.

In many everyday situations, both are possible. For learners, it is enough to know:

• 는 = topic/comment feel
• 가 = subject/focus feel

It usually implies some degree of contrast or surprise because of 생각보다.

So it is not just:

• This problem is easy. = 이 문제는 쉬워요.

It is more like:

• This problem is easier than I expected.
• This problem is easier than I thought.

So the speaker probably expected it to be harder.

No. 생각보다 can be used very widely with adjectives, descriptive verbs, and even some action-related expressions, as long as the meaning is more/less than expected.

Examples:

• 생각보다 맛있어요. = It’s tastier than I thought.
• 생각보다 커요. = It’s bigger than I thought.
• 생각보다 잘해요. = They do it better than I thought.
• 생각보다 오래 걸려요. = It takes longer than I thought.

So once you learn 생각보다, you can use it in many situations.

The sentence follows a very common Korean pattern:

• Topic + comparison phrase + descriptive verb

Breaking it down:

• 이 문제는 = as for this problem
• 생각보다 = than expected / than I thought
• 쉬워요 = is easy

So literally, Korean puts things in this order:

• This problem
  • than thought
    • is easy

This is normal in Korean because descriptive words like 쉬워요 usually come at the end of the sentence.