jungyohan bubune hyeonggwangpeneul sseumyeon najunge bokseuphagi swiwo.

Because 중요하다 is a descriptive verb (an adjective-like verb in Korean), not an action verb.

When a descriptive verb modifies a noun in the present state, it usually takes -ㄴ/은:

• 중요하다 → 중요한 = important
• 예쁘다 → 예쁜 = pretty
• 크다 → 큰 = big

So:

• 중요한 부분 = an important part / important parts

By contrast, -는 is typically used with action verbs:

• 먹다 → 먹는 사람 = a person who eats
• 공부하다 → 공부하는 학생 = a student who studies

So 중요하는 is not correct here.

부분 means part or section.

The particle 에 here marks the location/target where something is applied. In this sentence, it means something like:

• on the important parts
• to the important parts

So:

• 중요한 부분에 형광펜을 쓰면
  = if you use a highlighter on the important parts

This 에 is similar to the surface/location where the action happens.

Yes, 쓰다 can mean to write, but it also commonly means to use.

Here it means to use:

• 형광펜을 쓰다 = to use a highlighter
• 돈을 쓰다 = to spend money
• 약을 쓰다 = to use medicine

So 형광펜을 쓰면 means:

• if you use a highlighter

The particle 을 marks 형광펜 as the thing being used.

Yes, 형광펜으로 is also possible, but the nuance is slightly different.

• 형광펜을 쓰다 = to use a highlighter
• 형광펜으로 표시하다 / 줄을 긋다 = to mark / underline with a highlighter

So:

• 형광펜을 쓰면 focuses on using the tool
• 형광펜으로 focuses more on the means/instrument

Both are natural, but the given sentence is very normal Korean.

-면 means if or when.

It attaches to a verb stem to show a condition:

• 가다 → 가면 = if/when (someone) goes
• 먹다 → 먹으면 = if/when (someone) eats
• 쓰다 → 쓰면 = if/when (someone) uses

So:

• 형광펜을 쓰면 = if you use a highlighter

In everyday Korean, -면 often sounds like a practical if:

• If you do this, the result is that...

나중에 means later.

It tells you when the reviewing will be easy:

• 나중에 복습하기 쉬워
  = it’s easy to review later

So the sentence is saying that using a highlighter now helps with reviewing at a later time.

Position-wise, adverbs like 나중에 are flexible in Korean, but placing it before 복습하기 is very natural because it directly relates to the reviewing.

This is the very common pattern:

• Verb stem + -기 쉽다 = to be easy to do

Here:

• 복습하다 = to review
• 복습하기 = reviewing / to review
• 복습하기 쉽다 = to be easy to review

So:

• 나중에 복습하기 쉬워 = it’s easy to review later

Other examples:

• 읽기 쉬워 = easy to read
• 이해하기 쉬워 = easy to understand
• 찾기 쉬워 = easy to find

The -기 turns the verb into a noun-like form, so the whole phrase literally means something like reviewing is easy.

All three are related, but they differ in speech style.

• 쉽다 = dictionary form
• 쉬워요 = polite casual style
• 쉬워 = casual intimate style

So the sentence as given is in casual speech:

• 중요한 부분에 형광펜을 쓰면 나중에 복습하기 쉬워.

If you want a polite version, you could say:

• 중요한 부분에 형광펜을 쓰면 나중에 복습하기 쉬워요.

Why 쉬워 and not 쉽어?
Because 쉽다 is an irregular-looking adjective in conjugation:

• 쉽다 → 쉬워
• 덥다 → 더워
• 어렵다 → 어려워

Yes, the subject is omitted, which is very common in Korean.

The sentence does not explicitly say you, it, or one. Depending on context, it can mean:

• If you use a highlighter on the important parts, it’s easy to review later.
• Using a highlighter on the important parts makes it easy to review later.

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

So even though English usually needs a subject, Korean often does not.

Because the object is understood from context.

You are reviewing the material / notes / text / content you marked earlier, but Korean often omits that if it is obvious.

So 복습하기 쉬워 does not need to say exactly what is being reviewed. It just means:

• it becomes easy to review (it/the material) later

This kind of omission is extremely natural in Korean.

It can mean either, depending on context.

Korean nouns usually do not have to mark singular vs. plural the way English does.

So:

• 중요한 부분 can mean the important part
• or important parts

In this sentence, English would often translate it as the important parts, because that sounds more natural when talking about highlighting text. But the Korean itself does not force that distinction.

It most naturally refers to physically marking important parts with a highlighter, especially in notes, a book, or a printed handout.

That said, the broader idea is:

• marking important information helps later review

So the sentence is concrete, but the overall meaning can be applied more generally to studying and note-taking habits.

Yes. A few natural alternatives are:

• 중요한 부분에 형광펜으로 표시하면 나중에 복습하기 쉬워.
  If you mark the important parts with a highlighter, it’s easy to review later.

• 중요한 부분을 형광펜으로 표시해 두면 나중에 복습하기 쉬워.
  If you mark the important parts with a highlighter in advance, it’s easy to review later.

• 중요한 부분에 줄을 그어 두면 나중에 복습하기 쉬워.
  If you underline the important parts, it’s easy to review later.

The original sentence is still very natural and simple, especially for everyday advice.