I think it’s because with the words in -ly you know they’re supposed to refer to the noun? With a “stern looking man”, you might have doubts whether the guy is stern-looking or both “stern” and “looking” at something, because stern is an adjective. A sternly looking man can’t both be sternly and be looking.
I think it’s because with the words in -ly you know they’re supposed to refer to the noun? With a “stern looking man”, you might have doubts whether the guy is stern-looking or both “stern” and “looking” at something, because stern is an adjective. A sternly looking man can’t both be sternly and be looking.