"She is pretty but lazy."
Translation:그녀는 예쁜데 게을러요.
For this kind of phrase using "데" is like saying "but" also it can be like"however" saying "예쁜데" is softer sounding than "예쁜지만". Such as this phrase "좀 바쁜데 즐거웠어요" - (I'm) a little busy but (I) had fun/enjoyed." But it does have many uses, If you keep going with this course it will teach many different ways "데" is used. keep going :)