sonnimi ogi jeone geosireul kkalkkeumhage jeongrihae nohasseoyo.

I tidied up the living room and left it ready before the guest came.

Breakdown of sonnimi ogi jeone geosireul kkalkkeumhage jeongrihae nohasseoyo.

• ~이: subject particle
• ~을: object particle
• 오다: to come
• 거실: living room

Questions & Answers about sonnimi ogi jeone geosireul kkalkkeumhage jeongrihae nohasseoyo.

Why does 오다 become 오기 in 오기 전에?

Because -기 turns a verb into a noun-like form.

• 오다 = to come
• 오기 = coming

Then that noun-like form combines with 전 / 전에, which means before.

So:

• 오기 전에 = before coming / before someone comes

This is a very common pattern:

• 먹기 전에 = before eating
• 자기 전에 = before sleeping
• 출발하기 전에 = before departing

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Why is it 오기 전에, not 오는 전에 or 왔기 전에?

With before, Korean normally uses verb stem + -기 전에.

So the natural form is:

• 오다 → 오기 전에

Why not the others?

• 오는 전에 is not the normal pattern here.
• 왔기 전에 would sound like before having come, which usually does not fit the intended meaning.

The idea is that the action of coming had not happened yet at the time the living room was tidied. So Korean uses 오기 전에, not a past form.

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What is the role of 손님이 here? Why is it 이, not 은/는?