How should one phrase validation error messages?

Question

What is the best phrase to use considering that these are short error messages that is positioned right beside the labels / input they are referring to. For example, there's a missing email address, what is the best way to tell the user? (I omitted the label as it's redundant considering that it's already beside the label.)

1. Tell them what to do: "Enter a value."
2. Tell them what's wrong: "This cannot be left blank." OR "This field is missing."

Which of the 2 is the best approach? I'm aware that combining these two are good for general error messages (usually at the top of the form or where there's sufficient space). But if you only have little space (just one-line), which is better?

Answer 1

I use those format:

Fieldname cant be empty

or

Please complete fieldname

or

Please enter a valid fieldname

You can study in deep error messages here:

• Inline validation in web form
• Handling User Error with Care