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Each time you supply a value in a control, sends an asynchronous request to the server to validate the data. This means users will get immediate feedback while they are using your form. They do not have to wait until they click the form's submit button for example to discover that a zip code is missing a digit or that they neglected to fill in a required phone number field. Try to type letters in a number control, for example, and you’ll immediately see an error message.You'll want to test your form to make sure that you are using the correct control based on the validation you want. For example if you used a Text control for a phone number field, you'll won't see the correct error occur if you enter an incorrect phone number. Go back and edit your form to change the control type for that field from Text to Phone.
Your form 's submit button will be disabled csnnot be submitted until valid data is entered into all required fields and if data is entered into an optional field that data also must be valid. You will want to test your form to verify that all the fields that you want to require the user to fill are indeed set to required in the form. If the user does not enter data into all your form's required fields the submit button will remain greyed out until they do. This prevents the user from submitting a form that does not yet have all the data that you require. The submit button will also disabled if you enter or enters invalid data into an optional field, the invalid fields will be highlighted with an orange background color when the Submit button is clicked. For example, enter abcde into a Money control. An error will appear to alert the user that the entered data is invalid. And even if all required form fields are filled you'll see that the submit button is again greyed out the form cannot be submitted until the error is corrected. For example, replace abcde with 89.25,click Submit and the submit button can be clicked.If you click on the Submit button in a form that contains invalid data or required fields that are empty ( Submit button is greyed out), the message shown below will display. This alert form will be submitted.
The designer can also display a message instructing the user what to do. This method is very helpful to users when trying to determine why a form does not submit. 
Refer to this topic for the details.
Sometimes you'll test your form and see that the submit button remain grey'd out form will not submit even when all required fields appear to be filled with valid data. The cause is always a hidden required field with no data or a hidden field with invalid data. Since the field is hidden your users won't be able to enter a value into that required field nor to correct an invalid value in that field and will therefore never be able to submit your form. In a simple form it won't be hard for you to open it in the form designer, find the hidden required field and fix the problem. In a large form or one that has dynamic business logic that makes fields hidden/visible or required/optional or sets values in business rules, you'll need to debug this problem while testing the form.
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