As mentioned, the simplest way to submit a form is to fill in the form using the appropriate actions for this as described earlier.
Sometimes you need to loop through a form if you cannot get the desired result in a single form submission. Consider the book search example form. If you want to search for books in all available languages and for all reader ages, you cannot do this in a single form submission, because the site will not allow such a general search. Instead, you have to loop through the languages and the reader ages, and make a form submission for each combination of language and age. To do this you use the Loop Form actions: the Loop Field Values, For Each Option and For Each Radio Button. The Loop Form actions does not submit the form, so this must be done separately in a subsequent Click action that click on one of the submit buttons of the form. To loop over combination of field values you just place several steps with Loop Form actions after each other before the Click action that submits the form.
Use the Extract URL action on the submit button of the form, if you just want create a URL that represents a submission of the form.