To urge is to encourage someone, to push them forward to some action. To admonish is to counsel someone against some action, to advise them to not do something. So these two words are antonyms.
The opposite (antonym) of antonym is synonym. An antonym is a word that means the opposite. A synonym is a word that means the same thing, or nearly the same thing, for a given context.
Some words do not have direct opposites [antonyms]. Antonyms do not usually substitute for phrases. The phrase "you are" has no antonym. A contrasting version of "you are" is "you are not".