The framework will add the error messages defined in the model with DataAnnotation, but you can add custom validation errors:
ModelState.AddModelError("fieldname", "error message");
In the view you can add
Html.ValidationMessage("fieldname")
or ValidationMessageFor to the fields. Or use ValidationSummary somewhere near the form.
You can also use ViewBag to pass data between controller and view.
Be aware that if you use model generated from database, you need to use partial classes to add DataAnnotation.
Update:
You forget to share an important information with us: this action is called in an ajax backtalk. See this thread:
http://www.telerik.com/community/forums/aspnet-mvc/grid/how-to-return-error-information-to-grid-in-ajax-editing-mode.aspx[
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