덜 ~ Less

덜 is used as an adverb to mean ‘less’, the opposite to 더.

Examples
오늘은 어제보다 덜 더워요.
It is less hot today than yesterday.
난 언니보다 덜 예뻐요.
I’m less beautiful than my sister.
그 책보다 이 책 덜 어려워요
This book is not as hard as that book.