Retain The Field Information

Can anyone tell me on how to retain the information that was being key in previously and mark which field was being filled in incorrectly rather than reset all the field to previous value?

I have done checking like this but i think it is not quite user friendly:

function verifyBlank(thisForm){
if (thisForm.TeachersName.value == "none" || thisForm.Lab.value == 0 || thisForm.Class.value == 0 || thisForm.Subject.value == 0 || thisForm.PeriodFrom.value == 0 || thisForm.PeriodTo.value == 0 || thisForm.Day.value == "none" || thisForm.Month.value == "none" || thisForm.Year.value == "none")
{
alert('Incomplete information');
return false;
}
return true;
}