2) when ever we using DynaActionForm, how can we validate the inputs?
If you're using DynaActionForm and you want to validate, one way to do it would be to change to DynaValidatorForm and use the Struts Validation Framework to validate. DynaValidatorForm has all the features of DynaActionForm, but adds validation through the framework (If you're unfamiliar with the validation framework, just Google "Struts Validation Framework")
Of course you also have the option of validating by subclassing DynaActionForm and including a validate() method in your subclass, but if you're going to write a new class anyway, you might as well subclass ActionForm and put your properties in as well.
Another option is simply to do the validation in the Action class by populating an ActionMessages object and calling the addErrors() method.