This is the 11516th most frequent Korean word.
나가면
"If (someone) goes out" or "when (someone) goes out."
In this sentence, '나가면' illustrates a conditional clause and means 'if (you) go out'.
비가 올 것 같으니 나가면 안 돼요.
It seems it might rain, so you shouldn't go out.
Here, '나가면' implies a hypothetical condition referring to the cat potentially 'going out'.
문을 열면 고양이가 나가면 안 돼요.
When you open the door, the cat must not go out.
In this context, '나가면' represents a habitual condition meaning 'if (you) go outside'.