If you want a field to be displayed only when there is some sort of input in another field, you can set this up using a rule in your form.
Let’s take an example: we have a form where respondents have to provide their email address. If the email address contains some sort of input, then we also want to show the phone number field.
If Email address is not empty then show Phone number. Do not type in empty – simply leave that text box blank.