Dear Navi:

You wrote:

>Is there any difference between:
>1-Don’t talk to John before I am back.
>2-Don’t talk to John until I am back.

>Do either of these sentences imply that you should talk to John when I have come back? Does either one mean:
>”Wait till I am back and then do talk to John.”

They are similar, but there is a subtle difference. #2 sets a specific condition, #1 is a bit more general. #2 suggests that there is a specific condition that cannot be fulfilled until I return. #1 merely suggests timing.

For example, if I were leaving to find out some information that John would want to know, then I might say #2 because a certain condition (my finding out something more) would give a better reason for speaking to John.

#2 can suggest “Wait till I am back and then do talk to John” if the context has been established. Without a context, neither is imperative about talking to John, just about NOT talking to him. Neither really says what your sentence in quotation marks suggests.