geunyeoneun useul ttae pyojeongi jeongmal jayeonseureowoyo.

This is a very common Korean pattern: one noun is the topic, and another noun is the subject of the description.

• 그녀는 = as for her / she
• 표정이 = her facial expression is the thing being described as natural

So the structure is roughly:

• As for her,
• her expression is really natural
• when she smiles/laughs

In Korean, the topic marker 는 often sets the person or situation, while 이/가 marks what is actually being described.

So this sentence is not strange even though it has both markers. It is basically:

• As for her, when she smiles, her expression is really natural.

웃을 때 comes from:

• 웃다 = to smile / laugh
• verb stem: 웃-
• -을 때 = when

Because the stem ends in a consonant, Korean uses -을 때.

So:

• 먹다 → 먹을 때
• 웃다 → 웃을 때

If the verb stem ends in a vowel or ㄹ, you usually use -ㄹ 때 instead.

Examples:

• 가다 → 갈 때
• 오다 → 올 때
• 만들다 → 만들 때

So 웃을 때 literally means when [someone] smiles/laughs.

It can mean either, depending on context.

In Korean, 웃다 is often broader than English laugh. It can refer to:

• smiling
• laughing
• having a cheerful expression

In this sentence, because it talks about 표정 (facial expression), English learners often feel that when she smiles sounds more natural than when she laughs.

So depending on context, this could be understood as:

• when she smiles
• when she laughs
• when she is smiling

Korean is a little less rigid here than English.

Because Korean often leaves out possessive markers like 의 when the relationship is obvious.

Here, once you say 그녀는, it is already clear that 표정 refers to her expression. So 그녀의 is unnecessary.

That is why:

• 그녀는 웃을 때 표정이 정말 자연스러워요.

sounds natural.

If you said:

• 그녀는 웃을 때 그녀의 표정이 정말 자연스러워요.

it would still be understandable, but it sounds more explicit than necessary, and less natural in ordinary Korean.

Korean often prefers the simpler version when ownership or relationship is obvious from context.

표정 means facial expression or the look on someone’s face.

It does not just mean the physical face itself. It refers to the expression someone shows, such as:

• a happy expression
• a nervous expression
• a blank expression

So in this sentence, 표정이 정말 자연스러워요 means that her face looks natural in expression, not stiff, forced, or awkward.

A very natural English rendering would be something like:

• Her expression looks really natural when she smiles.
• She has a really natural expression when she smiles.

정말 means really, and it strengthens what comes after it.

So:

• 자연스러워요 = is natural
• 정말 자연스러워요 = is really natural

It adds emphasis, but it is still very common and natural in everyday speech.

Depending on tone, 정말 can sound:

• sincere
• impressed
• enthusiastic

In this sentence, it makes the speaker sound like they genuinely think her expression is very natural.

This is because 자연스럽다 follows the ㅂ irregular pattern.

The dictionary form is:

• 자연스럽다 = to be natural

When it is conjugated with -어요, the ㅂ changes in the usual irregular way:

• 자연스럽다
• 자연스러워요

This is the same pattern as:

• 아름답다 → 아름다워요
• 어렵다 → 어려워요
• 춥다 → 추워요

So 자연스러워요 is the correct polite present form.

-어요 is the standard polite speech ending.

So 자연스러워요 means:

• is natural in a polite, conversational style

This is the form you would use in everyday polite speech.

Compare:

• 자연스러워 = casual
• 자연스러워요 = polite
• 자연스럽습니다 = more formal

The sentence is polite, but it does not contain an honorific form for her. It is just normal polite speech directed to the listener.

It can mean either, depending on context.

-을 때 basically means when or at the time that. In real use, it can refer to:

• a specific occasion
• a general habit
• something that happens whenever that situation occurs

In this sentence, it most naturally sounds like a general statement:

• When she smiles, her expression is really natural.
• Whenever she smiles, her expression looks really natural.

So the sentence is probably describing her usual way of smiling, not just one single moment.

Grammatically, yes. But in everyday spoken Korean, 그녀 is often less common than English learners expect.

Native speakers often:

• omit the pronoun entirely
• use the person’s name
• use another context-based expression

So in real conversation, people might more naturally say:

• 웃을 때 표정이 정말 자연스러워요.

if it is already clear who they are talking about.

그녀 is more common in:

• written Korean
• translated writing
• stories
• situations where the speaker wants to make the subject explicit

So the sentence is correct, but in casual speech, dropping 그녀는 is often more natural.

The sentence follows a very typical Korean structure:

• 그녀는 = as for her
• 웃을 때 = when she smiles/laughs
• 표정이 = facial expression
• 정말 자연스러워요 = is really natural

So Korean places the time phrase before the main description, and the verb or adjective comes at the end.

A rough structural breakdown is:

• As for her
• when [she] smiles
• her expression
• is really natural

This may feel backwards compared with English, but it is very normal Korean word order.