You look beautiful. WHY NOT "YOU LOOK BEAUTIFULLY?"

Hi Newguest

The verb 'look' is used as a linking verb in that sentence, and that means the verb refers back to the subject. That's why the adjective is used.

Try doing a search of this site for "linking verbs" to learn more about this type of verb.
You are beautiful.
You look beautiful.
You look worried.
You seem worried.
You sound worried.
He sounds worried.
He sounds hoarse.
He is hoarse.
He is good.
He looks good.
He feels good.
He feels calm.
He appears calm.
They appear calm.


He appears calm from behind the screen. (He has a calm appearance as we look at him while seated behind the screen.)
He appears calmly from behind the screen. (He comes out from behind the screen in a calm manner.)

The dog smells good. (It has a good smell. We just gave it a bath.)
The dog smells well. (It can correctly distinguish many different smells.)

The man felt desperate for the key. (He had a feeling of panic. He absolutely had to find the key.)
The man felt desperately for the key. (He used his hands to feel around in an attempt to find the key. He did this in the manner of a desperate person -- a person experiencing panic.)