How to add an If-Then conditional statement for a Google Form (Google Apps Script)?

I have a simple Google form with two text entry fields and one checkbox. If the user selects the checkbox, I want to run another function that will send an email message to the user; otherwise I want to skip the function. I already have the script up and running, but my script is currently running the function to send an email for EVERY user - and I want to restrict the function from running only if the user selects the checkbox.