As a UI architect, I am always wondering about the best format to display errors and notifications to users.
Which of the following examples is the best format?
- Name is required
- Name cannot be empty
- Please enter name
I'd use something like, "You must enter your name".
It's always good to keep things short, but the most important thing is that the messages are easy to understand. I've worked in places where the designers don't like how the error messages look, that they take up too much space, but most users won't see the error messages, so I think it's a case where usability, or in this case, intelligibility, is more important than design.
I would go with 'Please enter name' because it indicates required action.
And I agree with puppybeard that you should use as much words as needed for user to understand the message correctly. I would also highlight the field with error and if possible set focus (cursor) to that field. It's a bit more work, but usually very helpful for the user.