Hi,
could you please explain a little better what you want?
E.g. if I understood you correctly, then this could be done on the following way:
1) there is a textfield with name vegatable
2) if someone click on submit button, then it checks if value of this field "vegetable" is "lettuce", if not form is not submitted and there is error message.
You want something like this?
Regards,
Tihana