geunyeoga useumyeon nado ttara usge dwae.

가 marks 그녀 as the subject of 웃으면.

So the structure is:

• 그녀가 웃으면 = if/when she smiles/laughs
• 나도 따라 웃게 돼 = I also end up smiling/laughing along

Using 가 is very natural here because it simply identifies who is doing the action in the conditional clause.

If you used 그녀는, it would sound more like setting up she as a topic or contrast, as in as for her, when she smiles... That is possible in some contexts, but 그녀가 is the most straightforward choice here.

-으면 is a conditional ending meaning if or when.

Here, 웃으면 comes from:

• 웃다 = to smile / to laugh
• 웃으면 = if/when (someone) smiles/laughs

In this sentence, it often feels closer to when or whenever rather than a strict hypothetical if, because it describes a natural repeated reaction:

• 그녀가 웃으면 = when/whenever she smiles

So this sentence can express a general tendency, not just one future event.

In Korean, 도 replaces other particles like 는 or 가 in many cases.

So:

• 나도 = I too / me too

You do not say 내가도 or 나는도.

Here 나도 means that I also do the action. The implied idea is:

• She smiles, and I also end up smiling along.

Even though 나도 does not have 가 or 는, it can still function as the subject from context.

따라 웃다 means to smile/laugh along, to join in smiling/laughing, or to smile/laugh because someone else is doing it.

The word 따라 comes from the idea of following. So the sense is not just I smile too, but more specifically:

• I follow her lead and smile/laugh too
• Her smile makes me smile too

It gives the sentence a warm, natural feeling of emotional contagion.

-게 되다 often means to come to, to end up, or to find oneself doing something.

So:

• 웃어 = I smile/laugh
• 웃게 돼 = I end up smiling/laughing / I find myself smiling/laughing

In this sentence, 웃게 돼 suggests that the reaction feels natural or almost involuntary. It is not just a plain statement of action; it has the nuance of:

• I can’t help smiling too
• That’s what ends up happening to me

This is why it sounds softer and more natural than a simple 웃어.

돼 is the contracted form of 되어.

So:

• 되다 = to become
• 웃게 되어 → often shortened to 웃게 돼

This contraction is extremely common and natural in everyday Korean.

A useful spelling reminder:

• 돼 = from 되어
• 되 by itself is not the same thing

So in this sentence, 웃게 돼 is the correct form.

It can mean either, depending on context.

Korean 웃다 covers both to smile and to laugh much more broadly than English does. In this sentence, either may be possible:

• When she smiles, I smile too
• When she laughs, I laugh too

If the context is gentle and affectionate, English speakers often translate it as smile. If the scene is more openly funny, laugh may fit better.

So the Korean itself leaves that slightly flexible.

It can be understood as a general tendency or repeated situation.

Because of -으면, the sentence often means:

• Whenever she smiles, I end up smiling too

So it does not have to refer to just one event. It can describe a pattern or a typical reaction.

If the surrounding context is about one particular moment, it could also refer to that moment. But by itself, it sounds very natural as a general truth about the speaker’s feelings.

Not always.

그녀 literally means she, but in everyday spoken Korean, people often avoid using a direct equivalent of English she. Instead, they may use:

• the person’s name
• a title
• 걔 = that girl / her (casual)
• 그 사람 = that person

그녀 is more common in:

• writing
• literary style
• translations from English
• song lyrics or poetic language

So the sentence is grammatically correct, but in casual real-life speech, a Korean speaker might use a name instead of 그녀.

돼 is in the plain/casual style.

So this sentence sounds like:

• casual speech
• inner thoughts
• narration
• an informal written tone

More polite versions would be:

• 그녀가 웃으면 나도 따라 웃게 돼요.
• 그녀가 웃으면 저도 따라 웃게 돼요. if the speaker wants to sound more polite and use 저 instead of 나

So the original sentence is natural in an informal setting.

Yes, that is close to the grammar, but it sounds unnatural in English.

A more literal breakdown is:

• 그녀가 = she
• 웃으면 = if/when she smiles
• 나도 = I too
• 따라 웃게 돼 = come to smile along / end up smiling along

That helps show the grammar, especially -게 되다. But in natural English, you would usually say something like:

• When she smiles, I end up smiling too.
• Whenever she smiles, I can’t help smiling too.
• When she laughs, I laugh along too.

So the literal version is useful for study, but not as a polished translation.