This is the 19148th most frequent Korean word.
아프면
"If (someone) is sick or in pain."
In this sentence, "아프면" represents a conditional situation where one is feeling discomfort or pain specifically in the knee.
무릎이 아프면 의사를 찾아가야 합니다.
If your knee hurts, you should visit a doctor.
Here, "아프면" implies a conditional state of being unwell or in discomfort.
아프면 충분히 쉬어야 회복할 수 있어요.
If you feel pain, you need to rest sufficiently to recover.
In this usage, "아프면" is part of a hypothetical situation where extreme cold might lead to adverse health effects.