Help with error messages string

I want to change some details in error message with this great module.
For an example:
Comment form with 2 fields: Name and Comment.
Comments field is required. If the user clicks on save button without add some text in comment field, the error message pops up and it says: Comment field is required.
I don't know how to change this string with string overrides module.

For this detail on comments "Submitted by (name of the user) on (the date)" I found that the right string is: "Submitted by !username on !datetime".

Please anybody explain details about finding strings for Drupal error messages.