Hello! This question is quite relatable. But it is both the same actually, they both mean "if" although sometimes the use of 'kapag' and 'kung' is based on what seems more appropriate. In your example sentence, if I would use the word 'kapag', I would write it in THAT form, however if I use the word 'kung', I would say " Kung mahangin, mahirap mangisda." hope this helped a bit
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binibining
