You are useless.
You are in my thoughts
Wie alt bist Du? means: how old are you?
"du bist doof" or "du bist blöd" or "du bist dämlich" are equivalent to "you are stupid"
It could be the beginning of a sentence concerning a female person. du bist eine ... - you are a ... It can also be an equivalent of the following expressions, said in resignation or exasperation to a female person, meaning: Du bist mir eine feine Freundin! - A fine friend you are! Na, du bist mir eine Expertin! - Some expert you are! It can also be said in admiration, meaning the exact opposite.
Bist du = are you (informal, singular)
Du bist eine hübsche Frau, du bist süß. = You're a pretty woman, you're sweet.
bist du in Ordnung = are you okay
du bist scharf = you are sharp
"Du bist faszinierend." "Du bist erstaunlich."
du bist durcheinander du bist verwirrt
"Du bist alles" means "You are everything".
Du bist die Ruh' = you are the peace.