Not quite - sad in your first sentence is an adjective describing your appearance where in your second sentence it would be an adverb describing the way you are looking at something.

Compare:
He looks sad. (adj)
The salad is fresh.(adj)

He looks sadly at his empty plate. (adv)
The salad is freshly prepared. (adv)

You look good without your glasses, even if you don't look (see) well without them.

"look" here has two different meanings -> "aussehen" & "sehen"

See http://en.wikipedia.org/wiki/List_of_English_...

Ich habe nur mehr eine "Faustregel" aus der Schulzeit im Hinterkopf. Bei Verben, die mit den "Sinnen" (riechen, schmecken, aussehen, tasten, fühlen, usw.) zu tun haben, wird kein Adverb verwendet.

Das ist nur grob - aber es hilft oft.

I think I see two possibilities for confusion.

In German and English a copula like is or look is followed by an adjective, but this can be confusing to German speakers because, like an adverb, the adjective is not inflected.

(1) "You look sad."
(2) "Du sieht traurig aus. "

In English, "Don't look sad." can be thought of as an imperative negated equivalent of (1).

However, an idiomatic German equivalent, "Schau nicht so traurig (drein)!" or "Guck nicht so traurig!", might be parsed differently (more like "He looked at her sadly").

It can't be that bad. If it makes you happy. Then why the hell are you so sad ...
Remember Sheryl Crow's song?? :-))

Thank you all very much. I feel for the present (I'm eleven years old and learn English at school) the explanations "aussehen - sehen" and "appearance - the way one looks at sth." do very well for me. And the Wiki list has made me shiver ... ;-) Am I to learn all this in future? So far I've only learnt that any form of to be takes the adjective as in two of Helmi's examples. Again many thanks.

If you're only eleven, you're doing very well! Don't worry and keep up the good work.

I don't think you really need to learn a list. Perhaps you can get a feeling for the language. We say "sadly" when it tells you something about the verb, and "sad" when it tells you something about the person (or thing).

For example:
The cat looks sad. = I think the cat is sad when I look at it.
The cat looks sadly at his empty bowl = The cat looks in a sad way at his bowl.

In most cases, you can rewrite sentences with such verbs so that they contain a form of "to be".

He seems happy = He seems to be happy
He looks happy = He looks as if he is happy
He feels happy = He feels that he is happy
etc.

Don't looks so sad = Don't look so much like you are sad