그대 is a very formal/poetic way of saying "you" and it's not used very commonly in daily life. You'll hear it alot in songs and books and usually refers to your significant other or somebody you love. 너 is an informal way of saying "you" and its used a lot in daily life. Usually around friends or people who are younger than you.

Hello sweety...I think... 그대 is ---> to old peple(more than me) , to love (i use 그대 to my love ;) ,some nobody (like poem..novel..and fictional character) 너 is --> friend, and young people(more than me ), never use old people!(that is point)

그대 is the formal way of saying "you" while 너 is the informal way of saying "you"