"펜은 칼보다 강하다."
Translation:The pen is stronger than the sword.
'보다' particle implies '더' by itself. When you use 보다, you don't have to use 더 (though nothing prevents you from doing so, if you want)
말은 빠르다 - horse is fast
말은 더 빠르다 - horse is faster. 더 is needed to change fast into faster
말은 개보다 삐르다 - horse is faster than a dog. 더 is not needed.