geunyeoui pyojeongeul bogo nado ttara usge dwae.

표정 means facial expression or look on someone’s face.
표정을 is 표정 + 을, the object marker, because 보다 means to see/look at and takes a direct object.

So:

• 표정 = expression
• 표정을 보다 = to see someone’s expression
• 표정을 보고 = seeing her expression / after seeing her expression

The -고 on 보고 connects this action to the next one. In this sentence, it can feel like after seeing or on seeing her expression.

의 marks possession, so 그녀의 표정 literally means her expression.

• 그녀 = she / her
• 그녀의 = her
• 그녀의 표정 = her facial expression

In Korean, possession is sometimes omitted when it is obvious, so in some contexts you might hear something like 그녀 표정 in casual speech, but 그녀의 표정 is the clearer and more standard form.

Also, in real everyday Korean, 그녀 is less common in conversation than in writing, songs, stories, or translated-style language. Native speakers often use the person’s name, 그 사람, or just leave it implied.

나도 means me too, I too, or I also.

It is:

• 나 = I / me
• 도 = also, too

So 나도 shows that the speaker is doing the same thing as someone else. In this sentence, the idea is that her expression makes the speaker laugh too.

Here, 따라 means something like following, along with, or in imitation of.

It comes from 따르다, which can mean to follow. In this sentence, 따라 웃다 means to laugh along or to laugh because someone else is laughing/smiling.

So 나도 따라 웃게 돼 has the feeling of:

• I end up laughing along too
• I can’t help but smile/laugh too

It does not necessarily mean the speaker is consciously copying her. It often means her expression is contagious.

웃게 돼 uses the grammar -게 되다, which often means:

• come to
• end up
• happen to
• become

So 웃게 돼 is not just I laugh. It has the nuance of I end up laughing, I come to laugh, or it makes me laugh.

Compare:

• 웃어 = I laugh / I’m laughing
• 웃게 돼 = I end up laughing / I come to laugh

In this sentence, 웃게 돼 suggests that seeing her expression naturally leads to that result. It feels less direct and more like an automatic reaction.

돼 is the contracted spoken/written form of 되어.

• 되다 → 되어 → 돼

So:

• 웃게 되어 = full form
• 웃게 돼 = contracted, natural everyday form

Both are correct, but 돼 is much more common in normal speech.

It can imply both sequence and cause, depending on context.

Literally, 보고 is seeing and... or after seeing. But in many sentences like this, English naturally translates it with a causal meaning:

• Seeing her expression, I end up smiling too
• When I see her expression, I smile too
• Her expression makes me smile too

So grammatically it is the -고 connector, but in meaning it can feel close to because I see her expression.

It depends on context.

웃다 can mean:

• to laugh
• to smile

Korean often uses 웃다 more broadly than English laugh. If the scene is gentle and emotional, English may translate it more naturally as smile. If it is more obviously funny, laugh works better.

So in this sentence, both may be possible depending on the situation:

• I end up smiling too
• I end up laughing too

The sentence breaks down like this:

• 그녀의 표정을 보고 = seeing her expression
• 나도 = I too
• 따라 = along/following
• 웃게 돼 = end up laughing/smiling

So the overall structure is:

[After seeing her expression], [I too] [end up laughing along].

Korean often places background information first, then the main result at the end. The final verb phrase 웃게 돼 carries the main conclusion of the sentence.

Yes. The given sentence is casual speech.

A polite version would be:

그녀의 표정을 보고 나도 따라 웃게 돼요.

Or, depending on style:

그녀의 표정을 보면 나도 따라 웃게 돼요.

The main difference is the ending:

• 돼 = casual
• 돼요 = polite

Everything else can stay basically the same.

Yes, that is very normal in Korean.

In 그녀의 표정을 보고, the subject of 보다 is not stated, but it is understood to be the speaker: when I see her expression.

Korean often omits subjects when they are obvious from context. So instead of saying:

• 내가 그녀의 표정을 보고...

it is more natural to leave 내가 out unless you need emphasis.

This kind of omission is extremely common and one of the biggest differences from English.