"Le" means : The "Un" means : A La différence tient dans le fait que l'article est défini ou non. Le professeur est un homme : The teacher (this one) is a man (general). Hope it helps!

just replace "a" (undefined article) in english by "un" for the male form, and "une" for the female form, and replace "the" (defined article) by "le" for the male and "la" for female