This is the 11586th most frequent Korean word.
나타나면
"If it appears" or "if [someone/something] shows up."
In this sentence, "나타나면" ('When [it] appears') expresses the condition of an opportunity appearing.
기회가 나타나면 주저하지 말고 잡으세요.
When an opportunity presents itself, do not hesitate and seize it.
Here, "나타나면" ('If [it] occurs') indicates the condition of a problem arising that requires a response.
문제가 나타나면 해결 방법을 찾아야 합니다.
If a problem arises, you must find a solution.
In this example, "나타나면" ('When [they] show up') describes the condition of someone's appearance prompting an action.