To express the conditional, use ~(으)면 :

추우면 에어컨을 끄세요 = If it’s cold, please turn off the air conditioner.

(추우면 = if cold. 에어컨 = air conditioner. 을 = object particle. 끄세요 = please turn off)

The usage for ~(으)면 can be used to express hypothetical events as well as events that are likely to happen. In English, ~(으)면 can also be used to mean “When A happens, do B” or “When A happens, B will happen” :

여름이 오면 에어컨을 틀어야해 = When summer comes, you must turn on the air conditioner.

(여름 = summer. 이 = subject marker. 오면 = if it comes. 에어컨 = air conditioner. 을 = object marker. 틀어야해 = must turn on.)

Unless an environmental disaster occurs, it’s very probable that the summer season will come in the near future , so here, it’s a matter of when, not if.

The conditional can also be expressed in the negative :

싫으면 먹지 마세요 = If you don’t like it, please don’t eat it.

(싫으면 = if you don’t like it. 먹지 마세요 = please don’t eat it)

아니면 can be used to mean “if not” :

나 아니면 누구를 선택할 거예요? = If not me, who will you choose?

(나 = me. 아니면 = if not. 누구 = who. 를 = object marker. 선택할 거예요 = will choose)

And 하지 않으면 can mean “if you don’t do” or “if you do not” :

민수가 수술을 하지 않으면 죽을 거예요 = If Minsu doesn’t have the surgery, he will die.

(민수 = Minsu. 가 = subject marker. 수술 = surgery. 을 = object marker. 하지 않으면 = does not. 죽을 거예요 = will die.)