왜냐하면
waenyahamyeon · TOPIK 1
because, the reason is
왜 (why) + 냐 (asks) + 하면 (if one says) = if you ask why...
Literally a built-in question and answer: 왜 means why, 냐 is a quoted-question ending, and 하면 is if (one) says. So 왜냐하면 = if you ask why, then... It usually pairs with a sentence ending in 때문이다 or 거든요.