Quote Originally Posted by bacco22 View Post
One more question, Could anybody explain me the structure of the following sentence:
ты сегодня выглядишь молодецом

why has been used молодецом?
As iCake pointed out, the correct spelling is "молодцом" (and, similarly, the genitive singular is молодца, not молодеца; dat. sing. is молодцу, not молодецу, etc.)

But to explain the grammar a little more: Russian typically uses the instrumental case after verbs that mean "to be, to become, to seem, to appear, to turn out to be," etc.

При праздновании бар-мицвы, еврейский мальчик произносит, "Сегодня, я становлюсь мужчиной!"
During the celebration of (his) bar mitzvah, a Jewish boy says, "Today, I become a man!"

Remember that мужчина ("a man") is an example of a masculine noun that is feminine in appearance. That's why the instrumental singular ending is -ой here, not -ом as in молодец --> молодцом.

Another example -- and notice that the past-tense verb is feminine (to agree with the nominative subject, not with the instrumental object, which is masculine here):

Эта "собака" оказалась волком!
That "dog" turned out to be a wolf!

BUT

Этот "волк" оказался собакой!
That "wolf" turned out to be a dog!


And finally, the original sentence:

Сегодня ты выглядишь молодцом.
You look excellent today. (Literally: "Today, you look like a truly excellent person.")