Submit buttons should be written as <input type="submit">. Other <button> tags inside a <form> tag must include the type="button" attribute if you do not want them treated as submit buttons for the form:
<button type="button">This doesn't submit the form</button>
Do not place instructions or additional options below the submit button. Screen reader users will not know that there is additional content they should wait to hear before submitting the form.
See Text Alternatives to Images and Icons for notes on using icons in buttons.