-(으)면
-(eu)myeon
Cross the threshold and the door opens: -(으)면
The picture Standing at a threshold. Only by stepping over it does the door beyond open. -(으)면 is a threshold not yet crossed, -(으)니까 a fact already crossed — rain falling outside right now is 비 오니까, rain that has not come yet is 비 오면.
Where the picture shows up in -(으)면
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the threshold that changes things
비가 오면 안 갈래요.
bi-ga o-myeon an gal-lae-yo
If it rains, I'd rather not go.
Standing in the doorway looking up at the sky, then pulling your shoe back off as the first drops fall.
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the door that opens every time
이 버튼을 누르면 문이 열려요.
i beo-teu-neul nu-reu-myeon mu-ni yeol-lyeo-yo
Press this button and the door opens.
Every time a finger touches the button, the two door panels split apart.
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cross it and we'll do it together
시간 있으면 같이 저녁 먹어요.
si-gan i-sseu-myeon ga-chi jeo-nyeok meo-geo-yo
If you're free, let's have dinner together.
Pointing at the empty evening slot in their calendar and tipping your chin toward the restaurant.
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the door still shut
제가 부자면 좋겠어요.
je-ga bu-ja-myeon jo-ke-sseo-yo
I wish I were rich.
Standing at a shut door, picturing the big house behind it. The threshold has not been crossed.
For wishes that aren't real, Korean often uses -(으)면 좋겠다 or -았으면 좋겠다.
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the door that opens each year
봄이 되면 꽃이 펴요.
bo-mi doe-myeon kko-chi pyeo-yo
When spring comes, the flowers bloom.
Year after year, on the same branch, the buds open as soon as the air turns warm.
All of it in one picture
-(으)면 is a single threshold. Cross the rain threshold and you end up not going; the button threshold opens the door every time it is pressed; cross the 'you're free' threshold and dinner together follows. Look at a threshold you have not crossed and you get 'I wish I were rich'; make it a threshold that comes back each year and you get blossoms every spring. So the clause before -(으)면 is not reporting a fact — it only sets up the spot that has to be crossed before the rest can open.