그런데
geureonde · TOPIK 1
but, by the way
그런 (like that) + 데 (but/and yet) = that being so, but...
그런데 is a contraction of 그러한데: 그러하다 (to be so) plus the ending -ㄴ데, which sets up background before a contrast or topic shift. So it literally says given that, which is why it covers both but and by the way.